To evaluate the performance parameters of n-on-p long-wave HgCdTe infrared focal plane array (IRFPA) detector at low luminous flux, a low-luminous test platform is set up in this paper. Firstly, the low luminous flux test platform is introduced and the dark current test results of the devices are analyzed. Then, the blackbody response performance of long-wave HgCdTe infrared detector in a low luminous flux is investigated on the low-luminous flux test platform. At last, the changes of performance parameters such as responsivity and band detectivity under different luminous fluxes are compared and analyzed, and the results of the influence of device operating temperature on device performance are given. The test results show that for the 10.8 μm@50 K MCT focal plane detector, the device band detectivity reaches a peak value of 1.5×10¹² cm·Hz^1/2·W^-1 under the conditions of device operating temperature of 50 K and a luminous flux density of 5.8×10¹⁴ph·s^-1·cm^-2, when the detectivity is no longer increased with the increase of integration time. The experimental calculation results and related parameters can provide a reference for the application of the detector.