In order to find out the contact pressure intensity distribution and its effects on the workepiece flatness error in plane polishing with retaining ring, the contact model and the boundary conditions are built up based on the axial symmetric elastic contact theory. The pressure intensity distribution is calculated and analyzed with the help of the Hankel transform theory and the constructing solution method of Chebyshev orthogonal polynomials. And then, the effects of the dimensionless ring-workpiece gap, the load ratio of the retaining ring and the dimensionless ring width on the contact pressure intensity distribution are obtained. Finally, based on the distribution theory of material removal volume, the effects of the pressure intensity distributions on the flatness errors of polished workpiece are also investigated experimentally. The results show that the contact pressure intensity distribution becomes more uniform and the better profile of polished workpiece can also be obtained, if the dimensionless ring-workpiece gap is reduced, the load ratio is selected as 0.6 to 0.85, and the dimensionless ring width is taken as 0.13 to 0.40.