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1 Introduction In recent years, driven by the strong demand for propylene derivatives such as polypropylene, the demand for propylene has been increasing year by year. There are mainly two traditional techniques for increasing propylene production: one is to achieve this by reducing the reaction severity in hydrocarbon steam cracking ； The second approach is to upgrade existing FCC units into catalytic cracking units by changing the catalysts, reaction conditions, and process flow. Due to the shortage of naphtha and light oil resources, the development of steam cracking in China is somewhat restricted. The technology for producing propylene through heavy oil catalytic cracking (i.e., deep catalytic cracking) offers advantages such as the use of heavier feedstocks, a high proportion of C3/C2 compounds in the product, and low production costs; it is therefore a suitable propylene production method for China’s national conditions. The existing processes for producing propylene via heavy oil catalytic cracking draw on the experience gained from mature FCC processes, and are primarily focused on areas such as catalyst development, process optimization, and the selection of operating parameters; fundamental research on the reaction chemistry has not yet been carried out in depth. Heavy oil catalytic cracking is a complex series of parallel reactions, and the degree of reaction has a significant impact on the product distribution. In this study, the cracking reaction was carried out on a laboratory-scale micro-fixed-bed reactor; the reaction depth was controlled by adjusting the amount of catalyst used, and the product distributions at different reaction depths were used to simulate the product distributions at various stages of the actual catalytic cracking process ； Furthermore, the reaction patterns during catalytic cracking are analyzed, and the pathways for the formation of propylene are explored, providing guidance for determining the operational conditions in the technology for producing propylene via heavy oil catalytic cracking, as well as for optimizing the process flow to enhance the propylene formation reaction. 2 The experiment used Daqing VGO as the heavy oil sample, and its properties are shown in Table 1. The catalyst is R0, a specialized catalyst for the catalytic production of propylene from heavy oil developed by the Petroleum Chemical Science Research Institute; its active components are transition-metal-modified five-ring high-silica zeolites and rare-earth superstable Y-type zeolites ； Before use, it was subjected to hydrothermal aging at 790°C and 100% steam for 17 hours; the microreactivity after aging was 57. The experimental setup is a conventional heavy oil micro-fixed-bed reactor. The experiment used reaction conditions of a reaction temperature of 600°C, an oil feed rate of 1.0 g, an oil feed time of 40 s, and a nitrogen purge flow rate of 60 mL/min; the degree of catalytic cracking was controlled by varying the amount of catalyst loaded. Table 1 Properties of Daqing VGO: Data density (20°C)/g·cm-1: 10.86; Residue (ω), %: 0.02; Element composition: ωw(C), %: 86.24; ω(H), %: 13.62; ω(N)/μg·g-1: 560; ω/S)/μg·g-1: 810; Group composition (ω), %: Saturated hydrocarbons 85.0%, Aromatics 12.0%, Resins 3.0%, Asphaltenes