Reducing the thermal stress of the functional module is one of the key goals in the process of design and fabrication of focal plane assembly. As a result the characterization and measurement of the deformation of the module at low temperature is required. Generally the thermal stress is characterized by indirect parameters such as the deformation in engineering. Based on this, a measurement method including its error analysis of laser interference used to acquire the deformation of the cooled infrared detector was discussed. This method overcomes the difficulty of temperature control and frosting inherent in the other method. It uses the interference pattern produced in the surface of the standard mirror by interference of coherent light to characterize the deformation of the sample. Tests show that this method can realize the measurement of deformation of real-time temperature changes. As a result an in situ measurement of the deformation of the FPA module packaged in the dewar is realized.