BEIJING—China is tempering its controversial one-child policy, allowing more couples to have a second child in a surprise concession over a much-disliked control that comes as the country faces a looming worker shortage.

Couples will be able to have two children if one spouse is an only child, according to a wide-ranging Communist Party reform blueprint issued Friday, the most significant adjustment in a policy that has defined Chinese family life for more than three decades and perhaps the most dramatic policy change out...