The mechanism of constant pressure filtration of particle suspension in a viscoelastic fluid is studied. Effects of operating conditions on the performance of filtration are discussed. At the beginning of a filtration, the elastic behavior of fluid plays an important role on the performance of filtration, which results in a more compact cake and increases filtration resistance. After a period of time, the elastic effect diminishes gradually, and the flow behavior of fluid transforms to power law due to the decrease of filtration rate. A numerical program based upon the continuity equation of cake compression and the filtration equation is designed for simulating the variations of local cake properties during a course of constant pressure filtration. The average cake porosity decreases with time at the beginning of filtration and then increases and approaches to a given value when the flow behavior transforms to power law. The calculated results of average cake porosity agree very well with the experimental data when the filter chamber is filled with cake.