An embedded monitoring system that was used to acquire partial discharge (PD) ultrasonic in transformer was designed based on electro-acoustic combined detection method. This on-line monitoring system could synchronously detect two circuits' current signals caused by transformer PD: multiplex pulses and 6 circuits' ultrasonic signals. 12 core optical fiber transmission was used in hardware to suppress electromagnetic interference in field monitoring and improve the sensitivity of measurement; as well as the modular design method was adopted in software to improve the data rate and reliability of system operation. For the on-line operation of three transformers in the underground substation, the measured data show that the on-line monitoring system can detect PD signal of transformer on time, which can provide lots of creditable data for the condition-based maintenance of transformers.