A transmitter, a base station device, and a method for aligning a signal output from a transmitter are provided. The transmitter is connected to a first antenna, and the first antenna detects a second test signal transmitted by a second antenna that is connected to another transmitter. The transmitter includes: a signal generating unit, which generates a first test signal; a directional coupler, which receives the first test signal and the second test signal; and a signal processing unit, which measures a timing difference between the first test signal and the second test signal, and uses the measured timing difference to control signal generation, so as to align a signal transmission delay between the two transmitters. Thus, closed-loop detection and an adaptive rectification mechanism for transmission signals of multiple transmitters can be implemented, and accuracy of aligning a signal at each transmitting antenna is improved.