Laser ablation micro-propulsion technology is the most promising field in laser propulsion. The specific impulse is the important evaluate indicator of the micro propulsion performance. It represents the impulse generation by consuming unit weight of the working medium. Its accurate measurement can not only help to analyze the mechanism of laser ablation matter, but also help the design of micro laser ablation thruster. This paper presents a measurement method, which can not only obtain the impulse by single pulsed laser ablation the working medium, but also measure the ablation mass, which can directly measure the specific impulse. The method is based on the model of torsion pendulum in vertical direction movement, which is the direction of impulse by pulsed laser ablation and the gravitation direction caused by the loss of the ablation mass, to obtain the corresponding impulse and ablative weight. The paper deduced the measuring principle, pointed out the error and analysed the design principle of the torsion balance, the results show that, according to the torsion angle change due to the impulse by pulsed laser ablation and the mass loss by laser ablation, the maximum angle can be used to calculate the impulse and the stable rotation angle can be used to calculate the loss weight when the torsion period is much greater than four times the width of a single pulse.