Free dispersion of natural gas in the air environment possibly produces explosion and subsequent fire, which has a fatal harm to people's life. The accidental leakage of natural gas pipeline is one of the main productions for natural gas in air environment, and laser technology is a method to detect natural gas pipeline leakage. In this study, the effect of leaking natural gas dispersion on the laser detection was numerically investigated. Four factors effect on natural gas dispersion and intensity of laser detection were also analyzed including leakage aperture, leakage velocity, detection distance, and leakage time. The results show that the spectral detection value decreases by 3.72% at high-speed leakage, 2.74% at medium-speed leakage, and 0.46% at low-speed leakage, when detection distance increases by 10% in the vertical direction of leak hole. Detection time and leak hole both has a large influence on laser detection intensity in high-speed leakage compared with two others. This research results can provide a help for the safe evaluation of gas pipeline emergency.