A mirror deformation detection system based on STM32 and using the capacitance detection chip Pcap01-AD was designed to solve the problems of small deformation, high change frequency, and difficulty of accurately measuring the microdeformation that were associated with a large aperture adaptive sub-mirror. First, according to the characteristics of the deformable mirror, which was driven by the voice coil motor, a system scheme for measuring the microdeformation of the mirror using the capacitance displacement sensor was proposed. Then, the hardware and software are designed for the measurement system. The hardware was constructed with the microcapacitance detection chip Pcap01, STM32F103 microcontroller (MCU) system, and power supply circuit. The software part included programming the lower computer in C language to realize digital signal acquisition of the capacitance, communication between the Pcap01-AD and MCU, data conversion of the upper computer and data processing, real-time display, and data storage. According to the structural characteristics of the large aperture telescope sub-mirror system, the experimental platform was designed to enable the conduction of several tests. The test results show that in the range of 50 μm, the measurement sensitivity is 200 pF/3 μm, and the system has high accuracy, small error, and high detection efficiency. Thus, this system can be used in adaptive mirror surface deformation detection and other small-displacement measurements.