Previous studies on the construction chronology of the South and the North grotto temples in Longdong, Gansu, have primarily relied on historical documents. Through an in-depth on-site investigation of both grottoes, this study reveals that the South Grotto Temple exhibits significant flaws in geological conditions, design, and construction techniques. In contrast, the North Grotto Temple demonstrates more meticulous site selection and superior engineering execution. Comprehensive analysis suggests that the South Grotto Temple was excavated first. However, due to insufficient experience in construction procedures and project management, irreparable engineering defects emerged. Consequently, the patron opted to carve a new grotto at a more favorable location, leading to the later construction of the North Grotto Temple.