The presence of the new frequency introduces more observations and degrees of freedom in data combination, which is beneficial for cycle-slip detection and repair. A novel real time cycle-slip detection and repair method based on undifferenced triple-frequency observations was proposed. This method employed two groups of geometry-free and ionosphere-free combinations of code and carrier phase observables, and constructed the third linearly independent detector for cycle-slip by eliminating the ionospheric delay variation between two consecutive epochs. The performance of the proposed method was validated with BeiDou triple-frequency observations with 30 s sampling interval. Results show that this method can effectively detect and correct the cycle slips on each frequency, even under high ionospheric activity condition. 