The invention discloses a portable ultrasonic online detection partial discharge diagnostic device which comprises a first independent ultrasonic detection probe, a second independent ultrasonic detection probe and an independent host processor. The first ultrasonic detection probe conducts first-time wide-range detection, and determines the approximate range of partial discharge, the second ultrasonic detection probe conducts second-time precision range detection, and signals detected in each detection process need to be sent to the host processor. Firstly, collected signal filtering and amplification are carried out through a filtering circuit and an amplification circuit, then, the amplified signals are sent to a digital potentiometer, the digital potentiometer sends the signals to an analog-digital conversion module for conversion, and finally the converted signals are sent to a central processing unit for processing. On one hand, the processed signals are sent to a display module for display so that a worker can conveniently check the partial discharge situation; on the other hand, the processed signals are released by earphones through a power amplifier module.