In this paper a thcrmodynamic model and a 2-D numerical computation model have been proposed for computing the discharge coefficient of connecting passage of swirl chamber diesel engine. The transient change of discharge coefficient via engine crank, angle is computed for the first time. The effects of some parameters (the connecting passagc area, angle, position and the piston top clearance, pressure ratio, flow direction) on discharge coefficient are studied and analysed. The model and its results of computation will have great importance for improving the property and computational precision for swirl chamber diesel engine.