The steady and transient characteristic of lateral photovoltaic effect in a-Si:H juncitions have been explored by theoretical analysis. The results show that the theoretical predictions are in good agreement with the experiments .It 1is worthy to note that two important parameters of the sample (i,e.the sheet resistance l/as of a-Si:H and the transit time T_m) evaluated according the theoretical relations with the conventional accepted mobility and conductivity of undoped a-Si:H are much larger than that deduced from the experimental data .Based on this with a reasonable analysis, we suggest that electrons in a-Si:H layer may transport parallel to the junction with a surprisingly high mobiliry.