A method for detecting an unbalance of a multi-port amplifier MPA intended to be on-board a satellite is presented. The MPA includes a plurality of paths, each path being configurable in gain and phase. The method includes transmitting a first test signal which is spread spectrum modulated from the first transmitting station to the first pathway, the first test signal being generated in at least the useful band of the first pathway; receiving by the second receiving station configured in frequency to receive signals transmitted by the second antenna connected to the second path of the MPA, the signals being likely to include a replica of the first test signal; detecting and measuring a power of received signals corresponding to a replica of the first test signal having leaked at the output of the second output port; computing at least one unbalance value of the MPA from the measurement of the power of the replica of the first test signal received in the second earth station.