A method and an apparatus for transmitting uplink data in a licensed-assisted access (LAA) system are provided. The method includes: receiving mode configuration indication information sent by a network device, where the mode configuration indication information is used to indicate that a synchronous hybrid automatic repeat request (HARQ) mode and an asynchronous HARQ mode are configured for uplink transmission of user equipment on an unlicensed carrier; and performing uplink transmission on the unlicensed carrier using the synchronous HARQ mode or the asynchronous HARQ mode and according to the mode configuration indication information.