A Side-Intake piston water jet propulsor for marine vehicles employs a concept of Side-Intake of water and generates thrust through piston's reciprocating motion in cylinder. The concept of Side-Intake makes intake openings on the side and near the discharging end of the cylinder wall to achieve that the piston separates the cylinder into a dry and a wet compartment during its movement. The dry compartment is kept at atmospheric or ambient pressure. As a result, the piston confronts air or ambient pressure instead of water during its recovering stroke for water intake. From hydrodynamics point of view, this characteristic of piston's recovering stroke through air achieves the same function as an oarsman recovering his oar through air. However, said propulsor achieves that under water or a vessel's water line.