Simply add Real in NSolve:

soln1[a_, b_, c_, \[Xi]_, \[Lambda]_, \[Phi]_, rn_] := 
  NSolve[\[Rho]^2 + 
      2 \[Rho] (a Sin[\[Theta]] Cos[\[Phi]] + 
         b Sin[\[Theta]] Sin[\[Phi]] + c Cos[\[Theta]]) + a^2 + b^2 + 
      c^2 - rn^2 == 
     0 && \[Rho] - \[Lambda] rn Cos[\[Theta] + \[Xi] Cos[\[Phi]]]^2 ==
      0, {\[Rho], \[Theta]}, Reals];
soln1[1/2, 1/2, 1/2, 1/2, 1/2, 1/10, 1]

Thanks for your suggestion. It has problems as follows. (1) The original soln1 gives the following results:

soln1[a_, b_, c_, \[Xi]_, \[Lambda]_, \[Phi]_, rn_] := 
  NSolve[\[Rho]^2 + 
      2 \[Rho] (a Sin[\[Theta]] Cos[\[Phi]] + 
         b Sin[\[Theta]] Sin[\[Phi]] + c Cos[\[Theta]]) + a^2 + b^2 + 
      c^2 - rn^2 == 
     0 && \[Rho] - \[Lambda] rn Cos[\[Theta] + \[Xi] Cos[\[Phi]]]^2 ==
      0, {\[Rho], \[Theta]}];
(*evaluation*)
In[8]:= ss = soln1[.5, .5, .5, .2, .5, .1, 1]

During evaluation of In[8]:= NSolve::ifun: Inverse functions are being used by NSolve, so some solutions may not be found; use Reduce for complete solution information.

Out[8]= {{\[Rho] -> -2.51366 - 3.61652 I, \[Theta] -> 
   1.8313 - 1.79756 I}, {\[Rho] -> -2.51366 + 3.61652 I, \[Theta] -> 
   1.8313 + 1.79756 I}, {\[Rho] -> -0.146207 - 
    0.0312241 I, \[Theta] -> -1.82025 + 
    0.520643 I}, {\[Rho] -> -0.146207 + 
    0.0312241 I, \[Theta] -> -1.82025 - 0.520643 I}, {\[Rho] -> 
   0.152974, \[Theta] -> 0.785684}, {\[Rho] -> 
   0.311045 - 0.210578 I, \[Theta] -> 
   2.25043 - 0.388346 I}, {\[Rho] -> 
   0.311045 + 0.210578 I, \[Theta] -> 
   2.25043 + 0.388346 I}, {\[Rho] -> 0.417435, \[Theta] -> -0.617471}}
NOTE: The two real solutions are: In[8]:= ss = soln1[.5, .5, .5, .2, .5, .1, 1]

During evaluation of In[8]:= NSolve::ifun: Inverse functions are being used by NSolve, so some solutions may not be found; use Reduce for complete solution information.

Out[8]= {{\[Rho] -> -2.51366 - 3.61652 I, \[Theta] -> 
   1.8313 - 1.79756 I}, {\[Rho] -> -2.51366 + 3.61652 I, \[Theta] -> 
   1.8313 + 1.79756 I}, {\[Rho] -> -0.146207 - 
    0.0312241 I, \[Theta] -> -1.82025 + 
    0.520643 I}, {\[Rho] -> -0.146207 + 
    0.0312241 I, \[Theta] -> -1.82025 - 0.520643 I}, {\[Rho] -> 
   0.152974, \[Theta] -> 0.785684}, {\[Rho] -> 
   0.311045 - 0.210578 I, \[Theta] -> 
   2.25043 - 0.388346 I}, {\[Rho] -> 
   0.311045 + 0.210578 I, \[Theta] -> 
   2.25043 + 0.388346 I}, {\[Rho] -> 0.417435, \[Theta] -> -0.617471}}

NOTE: The two real solutions are:  {\[Rho] -> 
   0.152974, \[Theta] -> 0.785684}
{\[Rho] -> 0.417435, \[Theta] -> -0.617471}
These are the two solution I want.
(2) Using your suggestion (I use soln2 to avoid conflict)
soln2[a_, b_, c_, \[Xi]_, \[Lambda]_, \[Phi]_, rn_] := 
  NSolve[\[Rho]^2 + 
      2 \[Rho] (a Sin[\[Theta]] Cos[\[Phi]] + 
         b Sin[\[Theta]] Sin[\[Phi]] + c Cos[\[Theta]]) + a^2 + b^2 + 
      c^2 - rn^2 == 
     0 && \[Rho] - \[Lambda] rn Cos[\[Theta] + \[Xi] Cos[\[Phi]]]^2 ==
      0, {\[Rho], \[Theta]}, Reals];
EVALUATE]
In[20]:= ss2 = soln2[1/2, 1/2, 1/2, 1/5, 1/2, 1/10, 1]

Out[20]= {{\[Rho] -> 
   ConditionalExpression[
    0.5 Cos[0.199001 + 1. (-0.617471 + 6.28319 C[1])]^2, 
    C[1] \[Element] Integers], \[Theta] -> 
   ConditionalExpression[1. (-0.617471 + 6.28319 C[1]), 
    C[1] \[Element] Integers]}, {\[Rho] -> 
   ConditionalExpression[
    0.5 Cos[0.199001 + 1. (0.785684 + 6.28319 C[1])]^2, 
    C[1] \[Element] Integers], \[Theta] -> 
   ConditionalExpression[1. (0.785684 + 6.28319 C[1]), 
    C[1] \[Element] Integers]}}
Please note: The two real solutions:  {\[Rho] -> 
   0.152974, \[Theta] -> 0.785684}
{\[Rho] -> 0.417435, \[Theta] -> -0.617471}

are no where to be found. Can you illuminate me on this issue? Thank you.

@Hong-Yee Chiu

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