By taking glycerol, ethanol, DT-3 and glycidyl azide polymer (GAP) all with carbon doped as typical liquid working substances, and under different incident laser energies, the ablation product and the splash of working substance induced by this ablation product are investigated, and the thrust evolution law is obtained. The results indicate that, the high speed splash process at the beginning of splash is the main stage of thrust formation. The subsequent splash process makes the mass flow rate increase, but makes less contribution to impulse coupling, which inversely degrades the overall thrust performance.