In laser measurement for aircraft position parameters, when laser-beam passes through gas field produced by aircraft, the beam curves due to different refractive index in gas fluid field, which influences measurement result. This paper analyses the distribution regularity of refraction index in gas fluid field based on aerodynamics theory, establishes the simplified mathematical model, presents the factors causing the measurement errors, calculates the errors of laser beam curve and corrects the measurement result. This method improves the accuracy of measurement system and benefits the system design. Experimental results show that the method can calculate the errors of measurement results effectively.