We explore the demographic factors contributing to China's unbalanced sex ratio at marriagable ages. We develop a stable population model of the sex ratio at marriagable ages, and compare a series of population projections with alternative underlying assumptions about the key demographic inputs. The stable population model demonstrates that several demographic factors interact to influence the sex ratio at marriagable ages, including the sex ratio at birth, population growth, the age gap of marriage partners, and the sex ratio of survival from birth to marriageable age. The population projections further demonstrate that policies that seek to reduce the sex ratio at birth and the age gap at marriage and, to a lesser extent, increase fertility would be most effective at alleviating the problem. But no demographic changes are likely to occur quickly enough to balance the sex ratio at marriagable ages in the near future.