A reliable and simple modified QuEChERS method with UPLC-MS/MS was developed for the simultaneous determination of six pesticides (dimethomorph, imidaclothiz, lufenuron, methoxyfenozide, pyridaben, spinetoram) and their metabolites in pak choi. Method validation indicated good linearity (R² ≥ 0.99), accuracy (recoveries of 75%−112%), sensitivity (limits of quantification, 0.002–0.01 mg kg⁻¹), and precision (relative standard deviations ≤ 21%), and matrix effects were −36–28%. The half-lives of the six pesticides in pak choi were 2.2–12 d under open field and greenhouse conditions. Considering the short growth cycle of pak choi, the terminal residue levels (0.046–7.8 mg kg⁻¹) and the relevant maximum residue limits (MRLs) of some countries, 5 d was recommended as the pre-harvest interval for the six pesticides on pak choi. Dietary risk assessment revealed that the risk quotients were 3.1%−58% for different gender and age groups in China, indicating none unacceptable public health risk for general population. The results showed that all the six pesticides degraded faster and the terminal residues were much lower under open field conditions than those under greenhouse conditions, which was mainly due to the influence of rainfall, sunlight and other environmental factors. This work was thus significant in assessing the dissipation fate and food safety risks of the six pesticides on pak choi and facilitated the establishment of maximum residue limits.