Abstract: In the paper, a 2D ring model is developed to simulate the actions of spacecharge field in an electron linac injector. The average field Ez and Er, generatedby a ring with the width Δr and thickness Δz on another ring, are derived. Theparticle motion equations with the space charge, field can be solved and somephysical parameters can be given such as beam emittance, output phase width andoutput energy spread and others. The computer program for beam dynamicscalculations of the injetor is introducted and some computations are given.