The invention discloses a method for flying and ground conversion of performance of a ramjet engine based on a ground direct connection test. The method comprises the following steps of obtaining a thrust 1 generated by an air heater; obtaining an engine thrust-time curve, so as to obtain a cold test thrust Fa and a hot test thrust Fb of the ramjet engine; performing configuration design on an airinlet of the ground ramjet engine to make a thrust 2 be equal to a thrust 6; performing a numerical simulation solution on a thrust 4 and a thrust 5 by software; and performing conversion to obtain the actual thrust F of the ramjet engine during flight according to the working principle of a ground direct test bench and the condition that 1, Fa, Fb and 2 obtained by the steps S1-S4 are equal to 6. Compared with an existing method for flying and ground conversion of the performance of the ramjet engine, the converting method provided by the invention simply considers the difference between theground thrust and the flight thrust from the perspective of the acting force without considering the conversion relationship between the force and the momentum, the method is more accurate in flightramjet engine actual thrust directly obtained through ground test conversion, and is simple and economical.