Focusing system is one of the most important components of laser thruster when it is also used as a nozzle at the same time. Such parameters change with generatrix equation of inner surface of focusing system as the shape and the size of ignition zone, nozzle length and the distance between ignition zone and inner surface of nozzle. All these parameters have effects on the propulsion performance of laser thruster. Four kinds of ignition modes are set up according to different nozzle structures. The results of numerical calculation and experiment are coincident. When injecting energy is not larger than 60 J and the diameter of nozzle exit is constant, the impulsion performance of laser thrusters with a parabolic focal point is found better than that of those owing a round one, and the coupling coefficient of the former is about 38.84×10-5 N·s/J. And the impulse coupling coefficient is not sensitive to nozzle length and the distance between ignition zone and inner surface of nozzle. The results are very useful to design nozzle structure of laser thruster that works in air-breathing mode.