A method for determining needle coke preparation process liquid phase polymerization time is as follows: a pretreated raw material is put into a reactor with the diameter of 40mm-60mm and the height of 120mm-200mm, the reactor is sealed, and filled with nitrogen, the pressure inside the reactor is maintained at 0.2-2MPa by a constant pressure valve; the reactor is heated, reactor heating mantle temperature is controlled at 450 DEG C-550 DEG C by a temperature controller, at the temperature, the temperature curve of the reaction material in the reactor is measured, and automatically recorded in computer software; and the temperature change of the reaction material in the reactor is observed, and the time when the material temperature begins to rise is, namely, the liquid phase polymerization time. The method has the advantage of high accuracy.