A refrigerator with 250W cooling power at 4.5K is developed in the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. This refrigerator operates stably and meets the design target after the test and commission. A dewar is uesed to store the liquid helium produced by the refrigerator. A electronic heater and a thermometer are placed in the bottom of the dewar to test the cooling power and the refrigerating temperature. This paper describes the investigation on the factors on the measuring stability of the refrigerating temperature.This work may contribute to developing the next refrigerator.