The regional stress state impacts on the effectiveness of microseismic monitoring of hydrofracturing. But the ideal stress state in the fracturing region is difficult to obtain because of the interlaced distribution form of fractures and complex stress fields. In this paper, the object is to study the impact of complexity of stress field on stress inversion. We first numerically simulate two series of focal mechanisms in a homogeneous stress field and a complex stress field, and then implement stress inversion by utilizing Vavryčuk’s iterative joint inversion method and do the error analysis. The results indicate that the uncertainty of focal mechanisms in homogeneous stress field has a slight influence on acquiring a stress model, but does impact on the error statistics of the slip direction. Also the slip deviation increases as the complexity of the stress field rises and the error is much bigger than that caused by uncertainty of focal mechanisms when stress inversion is in a complex stress field. In addition, part of the fault planes cannot meet the Mohr-Coulomb failure criterion with the condition of a single stress model inverted as the complexity of stress field reaches a high level, which means the inversion result is desirable. Two series of actual field data in the Daqing oil field and the Changning Block are chosen in this paper. These agree well with the numerical simulation and are applied to stress inversion and error analysis to verify the conclusions above.