A numerical study is presented for the electroosmotic flow (EOF) at the cross region in microfluidic chips. The distributions of the electric potential due to the electric double layer (EDL) and the external electric field are discussed and the calculation of the latter can give rough speculations on the flow tendencies in the channels during various operation modes. Simplification of the two-dimensional Navier-Stokes (N-S) equations is obtained by focusing on the solution of interior flows, and the numerical calculation results show good agreement with the experimental images. The sample leakage to the separation channel during the "float" sampling proved to be caused not only by the sample diffusion, but also by the weak extension of the sampling electric field. It is also verified that with suitable voltage configuration, the "pinch" sampling mode is better than the "float" mode in sample plug control.