The orientation of laser quenching tool to the workpiece surface influences the change of temperature field of quenching area, and then influences the uniformity of surface hardness of the workpiece. According to the structure character of 5-axis laser quenching machine, a rule is advanced that the normal vector of the transient laser quenching area must be always perpendicular to the coordinate plane XOY during laser quenching process. The method to determine the optimal workpiece gesture by the normal vectors of two planes determined by four boundary points in the laser quenching area is proposed. The algorithm to ascertain the four points is given. The model is established to assure the optimal workpiece gesture. Genetic algorithm is used to calculate the location angles of C-axis and A-axis for the optimal workpiece gerture. The gesture of workpiece with B splines curved surface is calculated to verify the feasibility of the model.