The technology of underwater cast-in-situ molded concrete was adopted for the launchway in the renovation engineering of a shipbuilding yard. Vessels were launched into the water by the air bag. The slope and flatness of the launchway was the key for guaranteeing the safe launching of vessels. Underwater molded concrete was previously used for slope protection, the requirement of which to the slope and flatness was lower that for the launchway. According to the characteristics of underwater molded concrete and the launchway for vessels, we kept strict quality control on the bed flattening, molded bag processing, molded bag laying and concrete construction, and put lots of hard work on the constructing details. As a result, a satisfactory effect was achieved.