In order to study the blocking effect of gel in ultra-low permeability fractured reservoir, the blocking strength of the gel to a single fracture and the failure characteristics of the gel in different fracture width are studied by indoor visual fracture model, and the blocking effect of the gel combined with the pre-crosslinked particles is evaluated. Experiments show that the blocking effect of single gel is better than that of combination of the gel and pre-crosslinked particle for single fracture. Only when a large number of pre-crosslinked particles are filled with a large number of fractures, and the width of fractures was greatly reduced, can the blocking effect of combination of the gel and pre-crosslinked particle be better than that of single gel. As the width of fracture increases, the gel in the fracture changed from overall ejection to central breakthrough after injected with water, and the blocking strength of the gel decreases. The experimental analysis concludes that due to the poor injectability of the pre-crosslinked particles, it is difficult to achieve large doses of injection, and gel shall be the main plugging agent to block the fracture, while improving the strength of the gel itself is the key to enhance the blocking strength of gel in the fracture. The results of the study provide a reference for optimising the process in fractured reservoir.