Multipath interference is a main source of GNSS positioning errors in real-time applications as the majority of the systematic errors can be effectively removed through modeling and differential technologies. Because multipath-error has time-varying characteristics closely related to the observation environment, it is difficult to detect and eliminate, especially in dynamic positioning. Because the multipath effect on the different frequencies is quite different, this paper introduces a new method to detect BeiDou GEO satellite multipath-error based on the frequency-difference data of the triple-frequency SNR observables, . Three-frequency SNR-based multipath detector was developed using the observables from the BDS GEO satellites in the open-sky environment. The GNSS data in the high-multipath environment was used to verify the reliability and validity of the new method, The results show that BeiDou GEO satellites are easily affected by the multipath-error, and ordinary methods cannot easily detect this error. The new method proposed in this paper effectively detectz GEO multipath error, and reduces or eliminates the weight of the observables based on a threshold. It can be applied in GNSS data quality control.