This article discusses the distribution of α particles in fusion reactor. We divide these particles into two parts: i.e., a high energy part and a thermolized part, and introduce the concept of slowing down density. A set of slowing down-diffusion equations satisfied by distribution function of α particles are then obtained. For the parameters of typical tokamak fusion reactor, the set of equations are solved analytically with simplified conditions. Thus the distribution of a particles and the rate of self heating are obtained. Finally,, the results computed by using different diffusion coeffecients are compared.