Abstract: A mechanical cryocooler is the main equipment used to provide the low-temperature environment required for an infrared detector. The temperature of the cold chamber is lower than that of the IRFPA as measured by the diode. The temperature difference is related to the temperature measurement position of the diode and heat transfer resistance. In this study, the influence of the diode position and difference in the heat transfer resistance on the actual temperature of the cold chamber is theoretically analyzed and experimentally verified. The influence of the actual temperature of the cryocooler cold chamber on its performance is further examined via theoretical derivation and experimental testing. During the testing of the performance of the cryocooler with the Dewar test, the following points should be considered. (1) An inherent temperature difference exists between the position of the diode and the cold chamber of the cryocooler. The greater the temperature difference, the lower the actual temperature of the cold chamber, and the worse the performance of the cryocooler and test Dewar. (2) The temperature difference between the diode and cold chamber of the cryocooler is affected by the thermal resistance of the Dewar module. The greater the thermal resistance, the greater the temperature difference and the lower the actual temperature of the cryocooler chamber. (3) The position of the diode affects the thermal resistance between the diode and cold chamber, thus affecting the temperature of the cold chamber of the cryocooler. The position of the diode should be fully considered in the design of the Dewar test to avoid the influence of simulation distortion on the evaluation of the cryocooler performance.