Tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation is used to measure the NH3 concentration at normal temperature and pressure near 1531.7nm in near-infrared region. The concentration of 25ppm can be obtained by 10m long-path cell，and concentration from 25ppm to 400ppm has good linear relationship with 2f signal. The effect of particles on 2f signal is discussed in this paper，and a solution of the problem using laser intensity linear fit to make the concentration measurement immune to particle disturbance is suggested.