A mid-infrared formaldehyde gas detection system was developed based on the coaxial mode-locked cavity enhanced absorption spectroscopy technology. In order to realize the detection of formaldehyde, an interband cascade laser with a center emission wavelength of 3.6 μm was used as the light source, and a high-precision F-P resonator was used as the gas cell. The laser frequency was tuned by an electro-optic modulator and was locked to the cavity resonance frequency by the Pound-Drever-Hall (PDH) technology. In order to suppress the interference caused by the external environment and improve the accuracy and anti-interference of the system, a dynamic PDH locking technique was adopted. In this technique, the cavity length was modulated by a low-frequency saw-tooth signal, to realize the modulation of the cavity resonant frequency near the gas absorption peak. An appropriate scanning range should be selected to ensure the laser and the cavity keep locking during scanning. The formaldehyde concentration can be calculated by the amplitude of the cavity transmission signal. Experiments were carried out to evaluate the performance of the system. The absorption spectrum measurement experiment verified the effectiveness of the system. The system calibration experiment results show that the amplitude of the cavity transmission signal exhibits a good linear relationship with the formaldehyde concentration in the range of 0~10 mL·L^-1. The Allan analysis of variance shows that the system minimum detection limit is 52.8 nL·L^-1 when the integration time is 1 s which can be reduced to 3.3 nL·L^-1 at an integration time of 14 s. In addition, the system sensitivity can be further improved by increasing the effective absorption path of the resonant cavity. The system has high sensitivity, fast response speed, good anti-interference and long-term stability, making it have broad application prospects in the detection of trace formaldehyde.