Dear Xuan: I had to make small changes in your code, because I was unable to get it to work. It appears that there is no city in world more populous than Australia. So I tryed with Panama and I found 180 cities.
AllCitys = CityData[];
PanamaP = CountryData["Panama", "Population"];
ACP = {#, CityData[#, "Population"]} & /@ AllCitys;
ACP1 = DeleteCases[ACP, {_, x_} /; MatchQ[Head[x], Missing] == True];
Select[ACP1, #[[2]] > PanamaP &][[;; , 1]]
