Tilt-rotor aircraft has capability of both vertical take-off in low-speed and cruise in high-speed, and therefore has been highly valued by aviation practitioner; Short take-off performance can increase more payload and get higher safety than vertical take-off, So it is one of the unique take-off methods for tilt-rotor aircraft. This paper deduced the concise and effective formula for distance of takeoff run between tilting angle of engine nacelles and thrust to weight ratio for tilt-rotor aircraft in the short take-off mode during conceptual design stage and finally the work of calculation and analysis was done to validate the formula. The result shows that the method is reasonable and credible, the conclusion can be generally applied to similar aircraft and has good engineering application value.