The ionosonde data in Chongqing, Lanzhou, Qujing and Lhasa and the IGS TEC data are used to study the variation on some days before Wenchuan earthquake of May 12, 2008. The results show that obvious enhancement appeared in several parameters and mainly in the south of the epicenter. The f_oF₁ and f_oE showed little abnormal increment and the maximal relative enhancement (4.5% and 9% respectively) occurred at Chongqing on May 10. The variation off_oEs, h′F and h_mF₂ were irregular. The h′F showed a bit enhancement mainly at night. The f_oF₂ and TEC showed abnormal enhancement from afternoon to sunset in two to three days ahead. The TEC enhancement also appeared in the magnetic conjugate area in the south hemisphere and drifted towards the magnetic equator. The equivalent electric field in F region estimated according to the experiential formula is coincident fairly well with the theoretically calculated one based on the abnormal electric field hypothesis by Pulinets.