Forecasting the warm-sector rainstorm of low vortex shear type is a difficult point in the rainstorm forecasting operation of Shandong Province. From 30 to 31 August 2021, a large range of warm-sector rainstorm occurred in the central and peninsula area of Shandong Province, but the forecasted rainfall intensity was weaker and affected area was smaller than the observed, resulting in the missing report of the rainstorm in a large scale. Based on numerical forecast products, conventional surface and upper-air observation data, Doppler radar data, we review the forecast errors of this warm-sector rainstorm event. The findings suggest that, during the forecasting process, the symmetric instability characteristics of atmosphere, the characteristics of warm front frontogenesis in the boundary layer, the vertical interactions of ultra-low level jet, low-level jet and upper-level jet, and the function of weak cold air in the boundary layer and middle layer failed to be judged completely by forecasters. In the case that the environmental field had changed, the model products in a short time in the previous period were still used as the basis for the precipitation correction of the model results. This may be the critical reason for the insufficient forecast of this warm-sector rainstorm process. In the future, when forecasting the similar warm-sector rainstorms, forecasters should comprehensively analyze the characteristics of conditional instability, convective instability and symmetric instability, and also pay attention to the characteristics of boundary layer θse dense zone and the warm front frontogenesis characteristics of the boundary layer. Moreover, the vertical three-dimensional structure of jet stream and the role of weak cold air at different heights should be considered, and the overestimation or underestimation of rainstorm forecast by the numerical models should be judged according to the environmental field features, and then reasonable dynamic model correction should be carried out.