Abstract: Based on the concept of fracture complexity index for vertical wells,the new one has been put forward and utilized in horizontal well’s multi-stage fracturing.The reconstructed concept of fracture complexity index was stretched to consider such factors as non-plain propagation in width,vertical fracture growth,sufficient reach of fracture length,and interference factor caused by induced stress between multi-stage fractures.What’s more,the range of the complexity index for different type of fractures was studied.Correspondingly,a systematic study was conducted focusing on how to maximize fracture complexity index in field multi-stage fracturing.The final result indicated that not only the favorable geology parameters but also optimized fracturing treatment parameters and filed control measures should be matched up maximize complexity index.Moreover,4 horizontal wells located in Sichuan Basin were conducted pilot test and obtained successful result.There were totally 65 stages (2-3 cluster for each stage) were hydraulically fractured and 140 separate fractures were available.By post analysis,the complex fractures may account for about 40% around.And some of the post-fracturing result was as well as expected.