Abstract: Zhang Zhao-xiang;Huang Kailin;Wang Yi When conducting mixed tubing-casing fractuting, the fracturing fluid flow simultaneously through the tubing and the annulus into the formation. Because the flowing resistance in the tubing is different from that in the annulus,the flow rate and flow velocity also vary. This paper analyzes in detail the effect of flow velocity variance on the mixed tubing -casing fracturing:the designed sand-adding procedure changed, made the designed sand (with high sand fluid ratio) following the fracturing fluid lose its proper efficacy,and the displacing liquid volume designed with conventional method was not suitable to the mixed tubing-casing fracturing mode. After pointing out the problems resulted from the mixed tubing-casing fracturing,this paper presents three kinds of improvement methods:sand settling in rat hole,nozzle adjustment,and time closing of valve.