Backfill empty phenomenon maybe happen after backfilling for underground penstocks of hydropower stations. The empty range and depth are important references for whether the underground penstocks need supplementary grouting. This paper selects representative pipelines of a certain project， and uses the three-dimensional non-linear finite element method to calculate the sensitivity of their stress conditions in terms of empty range and depth. Then the calculated results are compared with the resistance limit of the material to obtain the allowable empty range and depth of the steel pipe， and the impact of the backfill empty range and depth on the force of the steel pipe structure are scientifically evaluated.