The focal mechanism of earthquakes is an indicator of the variation of crustal stress, which is directly related to the occurrence of strong earthquakes. To date, no previous study has investigated the indication of focal mechanism change to the occurrence of normal-fault earthquakes with low apparent stress. This paper takes the 2021 M_W6.0 earthquake sequence in Crete, Greece, as an example to determine the relationship between stress variation and the occurrence of strong earthquakes by calculating the minimum spatial rotation angle between the earthquake sequence's focal mechanisms and the direction of the regional stress field. The focal mechanisms determined by multiple agencies were used to obtain the central focal mechanism as the focal mechanism of the earthquake, thus ensuring the accuracy of the focal mechanism. Then, the local stress field was determined by the earthquake sequence's accurate focal mechanism. Finally, the spatial rotation angle of the crustal stress field was calculated, and the relationship between the occurrence of strong earthquakes and the variation of the stress field was explored. Results indicate that the spatial rotation angle between the focal mechanism of aftershocks and the direction of the regional stress field is large in the short term after the mainshock, thereby corresponding to the weak seismicity of subsequent earthquakes with small magnitudes. Consequently, the spatial rotation angle decreases, corresponding to the occurrence of three strong aftershocks (M_W＞5.0). After that, the spatial rotation angle increases again for a long time, and the magnitude and frequency of the corresponding aftershocks decrease. Using the same 2021 M_W6.0 earthquake sequence as an example, the results reveal that the stress direction concentration also exists before the normal-fault earthquakes with low apparent stress. Therefore, this study provides an example for the exploration of earthquake stress precursors.