Hydraulic fracturing is a key technology for tight oil and gas development, and the fracturing pump injection rate is an important controllable factor during stimulation process. Variable-rate fracturing adjusts the fracturing pump rate dynamically to enhance the volume of fracturing treatment in tight reservoir. Focusing on the impact mechanism of variable-rate fracturing on the expansion morphology, a hydraulic fracturing model was constructed using the particle flow code (PFC) software. Three fracturing modes were designed and compared: constant-rate fracturing, step-increasing-rate fracturing, and alternating-rate variable fracturing. The models analyzed the influence of factors such as the change in injection rate, horizontal stress difference, and natural fractures on the extension of hydraulic fractures from a microscopic scale. The research results indicate that: (1) Under the constant-rate injection scheme, the main pattern of expansion is through a single fracture. Variable-rate fracturing modes are conducive to initiating branching fractures and increasing the range of fracture expansion, with alternating-rate fracturing showing the most significant effect. (2) High horizontal stress differences restrict the development of minor fractures, causing hydraulic fractures to directly propagate through natural fractures, thus reducing the effectiveness of variable-rate fracturing. Conversely, as horizontal stress differences decrease, both the number of fractures and their expansion length increase. (3) The presence of natural fractures facilitating communication and expansion between hydraulic fractures and natural fractures, thereby enhancing the complexity of the fracture networks. The research provides a theoretical basis and technical guidance for analyzing variable-rate fracturing patterns and optimizing fracturing schemes in tight reservoirs.