EPG simulation was applied to analysis the diffusion effect of two SSFP-FID signals, FISP and ES. The influence of T1, T2, and unbalanced gradient on signal intensity with consideration of diffusion effect was studied. The EPG simulation have a good consistency with the experimental data, indicating it can efficiently and precisely calculate the diffusion effect of SSFP signals. Both the simulation and phantom study reveals that for some specific tissues and imaging parameters, positive diffusion contrast can be obtained in FISP and ES sequence. For quantitative method based on SSFP signals, such as TESS relaxometry, the diffusion effect should be considered while large unbalanced gradients and small flip angle were employed for high resolution imaging in high field system.