The Yanchang Formation shale oil in Changqing Oilfield faces challenges such as short fracture propagation distance, low fracture network complexity, and easy inter-cluster flow, resulting in high initial production but rapid decline after fracturing. To address the engineering problem of insufficient fracturing due to high fluid loss, a numerical simulation model of multi-cluster hydraulic fracturing in horizontal wells was established. The main engineering parameters affecting the volume of fracturing were identified, and the effects of viscosity and flow rate on fracture propagation distance and overall fracturing volume were analyzed. The mechanical mechanisms of fracture initiation, propagation, competition, and fluid loss in multi-cluster fractures were revealed, and a method of alternating injection of variable viscosity fracturing fluids for fracture creation and propagation was proposed. Numerical simulation analysis show that increasing the viscosity of fracturing fluid increases the length of fractures but also increases the asymmetry of fracture lengths on both sides. Increasing the flow rate appropriately promotes an increase in the turning angle of fractures and the number of secondary microfractures, but excessively high flow rates could lead to uneven fracture propagation. When the viscosity of the fracturing fluid exceeds 70 mPa·s, fluid loss is significantly reduced, forming main fractures with large widths and simple shapes, thus increasing fracture length in the early stage of fracturing. When the viscosity is less than 30 mPa·s, fractures expand into multiple branches with inter-cluster flow near the well. Adopting an alternating injection strategy of variable viscosity fracturing fluids effectively increases the complexity of the fracture network and the overall fracturing effect. Specifically, high-viscosity fracturing fluids are first used to create the main flow channels, followed by low-viscosity fracturing fluids to create a complex fracture network, significantly increasing the fracturing volume. The research results provide theoretical basis and technical guidance for optimizing the fracturing design of horizontal wells in shale reservoirs, and are of great practical significance for the efficient development of shale oil.