With the continuous expansion of the array size of infrared focal plane detectors, the reliability failure of dewar caused by low-temperature thermal mismatch deformation of multi-layer structures is becoming increasingly prominent, and the demand for quantitative characterization of low-temperature deformation of the focal plane is becoming increasingly urgent. Based on the cold box component of the ultra-long linear infrared focal plane detector, a study was conducted on the low-temperature deformation of the focal plane, and theoretical simulation was conducted on the low-temperature deformation of the focal plane caused by multi-layer structure bonding. A test method for low temperature deformation of the focal plane of the detector at a working temperature of 90 K was designed. The results of the surface shape test were compared and analyzed with simulation calculations. The overall focal plane moved downward at low temperature, but the deformation curve was arched. The simulation showed that the chips on both sides were concave downward by about 924 m. In the middle position, the chip was concave downward by 136 m. The experimental results showed that the chips on both sides were concave downwards by about 40 m, and in the middle position, the chip was concave by 10 m. The data difference was related to the parameter setting of simulation materials. The rationality of the simulation results has been verified, which can provide a reference basis for the design of the focal plane multi-layer structure of ultra-long line array detectors.