Based on the located monitoring at greenhouse terrace in the typical loess plateau hilly-gully in Longquansi of Majiagou watershed in Ansai County, the erosion rules on terraces was studied. Results showed that the soil erosion of greenhouse terrace related to greenhouses area, rainfall, runoff and protective measures on terraces, and runoff played a dominant role. The critical monthly cumulative rainfall for erosion was 198 mm. The average amount of soil loss per unit area was less than 469.7 t/km² if the proportion of greenhouses area to terrace area was less than 0.5. Erosion control measures were proposed according to the erosion rules.