Abstract: In the area without sufficient electrical power system, internal combustion engines driving water pump system is popularly used in the agricultural irrigation, but it has disadvantages of long way power transmission, low efficiency and large space occupation. Integrating reciprocating combustion engine and plunger pump, internal combustion water pump(ICWP) has transferred the heat energy directly to the hydraulic pressure energy, its efficiency was improved and its configuration became compact. Working principle of the three-cylinder ICWP was expounded and its dynamics model was set up. On the basis of the above, the specific fuel consumption was simulated to find the best working area, and the area’s throttle opening was more than 35% and rotational velocity was 1 600-1 800 r/min. The result has been tested by experiments, and the feasibility of the computation model has been proved. Establishment of the model lays the foundation of further research on the structure of the three-cylinder ICWP.