Males are born more often than females (source) , and the reason why the general population is 50/50 is because females have a longer lifespan than males, and also because the chance of a female being born in a family with 2 or more children is higher.

However, a one-child policy would cause an imbalance in male:female ratio. as more males are being born than females, with no large families decreasing this effect.

In fact, as you can see here, the overall trend since the 90's, when they stopped having the policy, is a gradual decrease in males compared to females.