The necessity of cooling of InSb infrared detector and cooling temperature are described. The reasons why the cooling temperature of InSb focal plane detector is stricter than the single element detector are analyzed. It is shown that when the cooling temperature is 77～90K, the detectivity does not change very much. The single element detector still has higher signal-noise ratio at 137K, but signal-noise ratio of the focal plane detector drops greatly. The test results indicate that the focal plane detector using J-T cooler can achieve high performance.