China’s nuclear deterrent capability relies on so-called “first strike uncertainty,” which means letting the other side be unconfident of a completely successful first strike. But the fact that the Soviet Union conducted nuclear threat against China in 1969 showed that first strike uncertainty must be high enough to deter nuclear attack or nuclear threat. This article examines the threshold. Only after China deployed the DF-3 intermediate-range ballistic missiles in mid-1970s, the United States and the Soviet Union began to believe China had some nuclear retaliatory capability. Chinese leaders were not confident until China’s strategic missile forces gained independent launch capability in mid-1980s. U.S. missile defense is a potential destabilizing factor in Sino-U.S. strategic relations, both sides should work together in order to avoid negative security interaction.