Abstract: The maximum lift coefficient C_Lmax is directly connected to the selection of wing loading, which further affects aircraft's mass and economy. C_Lmax of propeller aircraft is improved with increased slipstream intensity at powered condition, however the conventional theory trends to some conservative and smaller power-off value, hence does not utilize the full potential performance of aircraft. Combining the airworthiness regulations and the actual flight condition of an aircraft with four turbo-prop engines, a new concept is proposed to find the optimal C_Lmax based on engine standby condition. The power-on C_Lmax is separated into four parts:power-off C_Lmax, propeller thrust component, propeller normal force and lift due to slipstream, which are simulated, measured and corrected by power-off and multi-balance power-on wind tunnel tests. The calculation shows that the propeller aircraft with four engines has slipstream intensity in the order of 0.1 at stall flight test condition with engines standby. The slipstream effect is the major factor of lift increment even at small slipstream intensity, and the increment increases as flap angle or slipstream intensity increase. The four engines at standby condition also generate considerable thrust and normal force due to lift. The power effect of turbo-prop engine increases the C_Lmax of different flap configurations up to 8%~9%. The C_Lmax obtained by the method agrees well with flight test, and the low speed performance of the aircraft is well utilized. The research can be an important reference for propeller aircraft design.