The performance of photoacoustic cell is one of the important factors which determine the trace gas detection system sensitivity based on photoacoustic spectroscopy (PAS). The photoacoustic cell design is a key part of PAS gas detection system. A first-order longitudinal length adjustable feedback resonant photoacoustic cell is designed based on PAS and acoustic theory. The resonance frequency is conveniently achieved by actual measurement, which effectively avoids the double counting process and the manufacturing error, makes the measurement easier and improves the sensitivity of detection system. The PAS trace gas detection system is built. The impacts of cavity length, photoacoustic cell length and source power on photoacoustic signal are discussed experimentally. The signal to noise ratio is 39.5 and sensitivity is 2.78×10-6.