Balise transmission module(BTM) is an absolutely necessarily device of onboard sub-system in China train control system(CTCS), to fulfill the communication between train and ground. In view of loss points occasionally occur in transmission which was unable to receive ground balise telegram signal, a balise onboard test platform was designed and developed. The principle, software and hardware architecture of the system were described. The test results showed that the platform could achieve the check of the main parameters of up and down links during balise transmission.