As I remember for other types of software, I can do a loop to repeat the equation until it satisfy a current condition.

Why can't you use a While ?

tStop = 10;
t = 0;
keepLooping = True;

While[ keepLooping,
 t++;
 If[t >= tStop,
  keepLooping = False,
  Print["I am still looping !"]
  ]
 ]

Well, I tried FixedPoint[], but it only gives what was in the function only

Below is an example of my equation, which is also from another thread http://community.wolfram.com/groups/-/m/t/421465

(*Temperature of water*)
Twater = 500
Cwater = QuantityMagnitude[
  ThermodynamicData["Water", 
   "ThermalConductivity", {"Temperature" -> 
     Quantity[Twater, "Kelvins"]}]]

ClassicalRungeKuttaCoefficients[4, prec_] := 
  With[{amat = {{1/2}, {0, 1/2}, {0, 0, 1}}, 
    bvec = {1/6, 1/3, 1/3, 1/6}, cvec = {1/2, 1/2, 1}}, 
   N[{amat, bvec, cvec}, prec]];

f = ParametricNDSolveValue[{Derivative[1][y][x] == 
     Piecewise[{{(y[x] + x^3 + 3 z - 18000*Cwater), 
        0 <= x <= 1}, {(y[x] + x^2 + 2 z), 
        1 <= x <= 2}, {(y[x] + x + z), 2 <= x <= 3}}], y[0] == 0}, 
   y[3.], {x, 0., 3.}, z, 
   Method -> {"ExplicitRungeKutta", "DifferenceOrder" -> 4, 
     "Coefficients" -> ClassicalRungeKuttaCoefficients}, 
   StartingStepSize -> 1/10];
point = {z /. FindRoot[f[z] == 100., {z, 1}, Evaluated -> False], 
   100.};
Show[ListPlot[Evaluate[{#, f[#]} & /@ Range[60., 100., 0.05]], 
  Joined -> True, Frame -> True, FrameLabel -> {"Z", "Y[3]"}], 
 Graphics[{PointSize[Large], Red, Point[point]}], ImageSize -> 400]

FindRoot[f[z] == 100., {z, 1}, Evaluated -> False]

Tnew = 170 + 2*z /. FindRoot[f[z] == 100., {z, 1}, Evaluated -> False]

If i use FixedPoint, how do I get Tnew to replace Twater and repeat again until it stop when Tnew=Twater?