The price discovery power of financial assets is a key component of national economic sovereignty, which is related to the market order and the security of national wealth. From the historical experience, once the development of onshore market is sluggish or over-regulated, the competitive offshore market will take advantage of the opportunity to develop rapidly. Based on the restriction events in domestic stock index futures in 2015 as a natural experiment, this paper analyzes the price linkage relationship between A50 in SGX(Singapore Exchange Ltd) and the domestic CSI 300 stock index futures. The results show that, after the domestic market is limited, the position of A50 stock index futures shows an obvious upward trend, and the demand for hedging increases. The rise and fall of A50 night and pre-market stock index futures trading can effectively predict the opening trend of CSI 300 index. During the synchronous trading period, the contribution of CSI 300 stock index futures in price discovery is 64.4%, which is still significantly higher than that of A50 stock index futures. After the domestic circuit breaker triggers, the trading volume of the A50 stock index futures has not decreased significantly, which indicates that the onshore market suspension can not significantly restrict the price discovery ability of the offshore market.