Abstract: Suspended sidetracking technology is based on some old wellbores, deviating from the old borehole from the sidetracking point and drilling the new borehole. Conventional suspended sidetracking is mainly used for sidetracking with the same size bit. Under-size bits are rarely used for drilling inside large boreholes, and the signs of successful sidetracking are slightly different. Analysis and research on the difficulties of under size bit sidetracking and drilling parameters, motor build-up rate and formation pressure test during sidetracking process, the success marks of under size bit, without trajectory reservation, short distance and conventional motor drilling tools suspended sidetracking are summarized, and the problems of under sized bit suspended sidetracking are solved. The successful implementation of sidetracking provides a technical reference for subsequent unconventional sidetracking operations.