Dashen CP302 is the first deep slim hole under-balanced casing window sidetracking horizontal well which is independently designed and constructed in Daqing oilfield, and has concentrated all the technical characteristics and the difficulties of deep well, slim hole well, under-balanced well, casing window and horizontal well. By optimization on the technical scheme, whipstock with duplex milling and cone milling window is determined. The paper introduces casing sidetracking technology and well trajectory control technology in detail.