Abstract: A dimensionless factor of conical nozzle is defined combined with incident laser energy, energy deposit efficiency, ambient air pressure, generatrix length and semi-apex angle of nozzle, and it is extended to the study of parabolic nozzle. Effects on impulse coupling coefficients of conical and parabolic nozzles under four semi-apex angles are calculated by radiation gas-dynamics computational code. Results indicate that, there are the maximum impulse coupling coefficients of the two nozzles, and the corresponding dimensionless factor is about 0.4. Impulse coupling coefficient increases with the decrease of semi-apex angle at same dimensionless factor, and the impulse coupling coefficient of parabolic nozzle is greater than that of conical nozzle under the same conditions.