The short takeoff and landing aircraft using distributed electric propulsion is a novel concept, for which the design method of its key parameters needs to be explored. In this paper, taking the conventional turboprop aircraft retrofitted with distributed electric propellers as an illustrative aircraft, the design problem of the aircraft key parameters is formulated as an optimization problem, and then the design optimization process for the key aircraft parameters is presented using the optimization method with surrogate model. Multidisciplinary analysis for the such aircraft conceptual design is used to evaluate the values of objective and constraint functions in optimization problems. The multidisciplinary analysis consists of several analysis modules including geometry, aerodynamics, propulsion, weight and performance. After optimization for the key parameters of the illustrative aircraft concept, the demanding requirements for the ground roll distance during takeoff and landing can be met at the minimum weight penalty. The ground roll distance during takeoff is less than 100m; the ground roll distance during landing is less than 70m. The optimal results also indicate that the distributed propellers are installed beneath the wing at a certain angle of inclination with moderate rotation speed and diameter, the wing area is increased slightly and required power for the turboprop engine is reduced significantly.