A system based on quartz enhanced photo- acoustic spectroscopy technique is investigated for trace ammonia gas detection. And hollow- core photonic band- gap fiber is introduced as reference gas cavity for high accuracy and sensitivity. The reference gas cavity is consisted with a 5 m long hollow- core photonic bandgap fiber, which filles with ammonia gas and splices with single-mode optical fibers at both ends. Modes interference is analyzed to obtain transmission spectrum with low interference noise. For improving detection accuracy, gas filling time and pressure are controlled in filling procedure. Ammonia absorption line width in ref? erence gas cavity of hollow-core photonic bandgap fiber is measured, and compared with the high-resolution transmission (HITRAN) database data. By this method, wavelength of distributed feed back (DFB) laser is locked accurately. Quartz enhanced photo- acoustic spectroscopy system is used for ammonia detection with optimal modulation parameters, which yields that a noise equivalent concentration (namely volume fraction) of ammonia is 6.74×10-6 (3 s).