In order to accurately calculate the cation exchange capacity Q_v of shaly sand reservoirs, a Q_v calculating method using NMR logging data was presented, a correlation model between Q_v and synthesized index was established. At the same time the data of four wells at the southern margin of Junggar Basin were processed using Q_v calculated by three different methods of NMR logging, natural gamma ray logging and spontaneous potential logging. It is showed that Q_v can be simply and efficiently calculated by these three methods in thick layers of shaly sand; in thin alternating layers of shale and sandstone, Q_v calculated using NMR logging is more reliable than that by the other two methods.