A method for transmitting uplink data, a user equipment and a network-side device are provided according to the present disclosure. The method for transmitting uplink data includes: transmitting a first message to a network-side device; receiving a second message fed back by the network-side device, where the second message is determined by the network-side device based on information about the network-side device and the first message; and transmitting the uplink data in an uplink data transmission manner corresponding to the second message. In solutions of the present disclosure, the user equipment can flexibly transmit uplink data in response to an instruction from a network-side device without being limited to transmitting the uplink data in an uplink scheduling grant-based manner, thereby achieving the objectives of fulfilling service requirements and transmitting uplink data with high flexibility and stability.