Six different MIL-88A (Fe) samples were synthesized by one-step solvothermal method for different reaction times (named M-4, M-8, M-12, M-16, M-20, M-24) and characterized by SEM, XRD, FT-IR, BET, UV-vis DRS, XPS and EIS. A variety of MIL-88A (Fe) catalytically activated peroxymonosulfate was used to degrade Rhodamine B solution under xenon lamp irradiation (500 W), as a simulated sunlight. Meanwhile the effect of solvent heating time on its catalytic performance was studied, and the reaction mechanism and influencing factors were analyzed. The results show that the variation of crystal morphology follows a process of “nucleation-aggregation-solution-recrystallization” with the increase of solvothermal time. The morphology of MIL-88A (Fe) is greatly affected by different solvent thermal duration. Fe―O clusters are formed between inorganic metal and carboxyl group of fumaric acid, and the REDOX conversion between Fe³⁺ and Fe²⁺ is active. The catalyst M-12 crystal prepared with a solvothermal duration of 12 h has the largest aspect ratio (3.33). There are more defects and unsaturated coordination sites at the junction of iron ions and organic ligands on its surface, and it has the strongest photocurrent response, the largest photogenerated electron-hole separation efficiency, the lowest electron transfer resistance and the highest conductivity, therewith, the best photocatalytic activity. In the M-12/PMS/artificial sun light system, the degradation rate of RhB (20 mg/L) reached 99.33% within 60 min, the system has a wide pH adaptation range and low influence of inorganic anions, moreover, ·OH, SO{}_{\mathrm{4}}^{•}^- and h⁺ are the main active free radicals, while ¹O₂ and ·O{}_{\mathrm{2}}^{-} play a role in assisting RhB degradation. The active free radicals are derived from three pathways: direct activation, direct activation by electron transfer and indirect activation by electron transfer. In the cyclic experiments, M-12/PMS/Light system has maintained an efficient removal rate of more than 90% for RhB, the sample structure and catalytic performance are stable, and the reusability is high, which has a good application prospect.