Terahertz time-domain spectroscopy (THz-TDS) technology has attracted enormous research interest in recent years due to its advantages of large dynamic range, high sensitivity and signal-noise rate (SNR), etc. However, the strong absorption of water vapor to terahertz wave has great impact on gas concentration demodulation results. In this paper, a quantitative analysis of different concentrations of ammonia is conducted on using THz-TDS at room temperature and pressure with the presence of water vapor in the measurement system. We propose and demonstrate a ratio demodulation method for the ammonia concentration and it effectively reduces the influence of water vapor compared with the difference method and the humidity compensation method. The experimental results show that the detection sensitivity increases to 33.0 mg/m3 and the concentration retrieval error can be controlled within 48.2 mg/m3 when the interaction length between the gas and THz wave is 96 mm. The concentration retrieval error is reduced by 84.6% and 77.4% compared with the difference method and the humidity compensation method respectively.