Based on the jet-flow theory, we establish a new method for calculating trajectory and extension of liquid jet in air by inducing the entrainment assumption and momentum analysis method, which is called jet-momentum model. We validate the new model and compare it with field test cases from bow-spray, pipeline-blowing, and calculation results of certain existing models. The results show that the trajectory calculated by the jet-momentum model is equivalent to the existing models, and is basically consistent with the measured values; the calculated extension range is also in well agreement with the measured values, and the jet-momentum model has wider application scenarios than the existing models. According to the working characteristics of bow-spray and pipeline-blowing, values of the two important parameters in jet-momentum model, jet expansion coefficient and velocity coefficient of inhaled air are recommended.