In order to estimate power of light aircraft quickly and accurately and to decide engine model at the beginning of the concept design phase, design target and airworthiness need to be satisfied. According to performance parameter and engine information statistics of multiple light aircrafts, empirical formula and functional relationship between parameters are concluded, we can calculate the internal values of power . The result show that consideration of design target and airworthiness elicit the final intersection of value, which is useful for applicant engine selection