Heavy ozone (O3) pollution is often observed in chemically industrialized Lanzhou and other capital cities located in the semi-arid and mountainous provinces of northwestern China. There are large knowledge gaps regarding the relationship between radical budgets and photochemistry in these cities. To gain insights into this relationship, a photochemical box model based on the Master Chemical Mechanism (MCM v3.3) was applied to investigate oxidative capacity and radical chemistry in the city of Lanzhou. The budgets of ROx (OH + HO2 + RO2) radicals were quantified, and the initiation, propagation, and termination process of the diurnal variation in the ozone chemistry were examined. The MCM model was constrained by in situ measurements at two sampling sites in the city, one located in the City downtown area (S1) and the other in the heavy petrochemical industrial area (S2) in the western suburb of the City, characterized by significant differences in volatile organic compounds (VOCs) and NOx concentrations between the two sites. Results showed that during the high O3 episodes in summer, OH initiation was dominated by the reaction of excited oxygen atoms O(1D) with water and the photolysis of nitrous acid (HONO) at the S1 site. At the S2 site, the most important production of OH was the reaction of O(1D) + H2O, followed by the reaction of O3 with VOCs. HONO photolysis was mostly identified at 7:00–13:00 local time at the S2 site, which is less important than that at the S1 site during the daytime. The photolysis of HCHO and OVOCs (except for HCHO) were the primary sources contributing to the initiation of HO2 and RO2 radicals at both sites. Results also revealed that the ROx termination could be attributed to the reactions of ROx with NO and NO2. The self-reactions between radicals also played an essential role at the S2 site. Overall OH was found to be the predominant oxidant, and NO3 was a major oxidant in the nighttime chemistry in the city.