The invention relates to a method for determining the bottom heat flow of a rocket by a radiation form factor, belongs to the technical field of the thermal protection design of a rocket thermal environment, and mainly relates to a method for determining bottom jet radiation heat flow when a carrier rocket and a liquid missile are in a rising flight phase. The method adopts a circular arc approximation method for the calculation of a jet boundary. A result obtained by the method is favourably matched with an experiment result when a ratio of jet pressure to external pressure is big. The bottom surface of the rocket and a jet boundary surface are subjected to mesh generation so as to obtain the radiation heat flow estimation values of different positions of the bottom of the rocket. Compared with the traditional single heat flow condition, the method is refined. A conservative structure heat-proof design is reduced, the installation position requirements of an instrument cable are lower, and a contribution is made to the weight reduction and the reasonable layout of the whole rocket.