The invention relates to an online fault diagnostic apparatus for a circuit board. The online fault diagnostic apparatus comprises an upper computer and a lower computer, wherein the upper computer is used for performing online signal processing and fault diagnosis, the lower computer is used for providing needed signals for a tested circuit board and uploading test information, and the upper computer and the lower computer are connected and communicated by using a serial bus. The lower computer comprises a CPU (Central Processing Unit) board, a switching power board, a relay switch matrix board, an analog switch matrix board, a digital signal board, A/D (Analog to Digital) and D/A (Digital to Analog) boards, an interface device and a detector, wherein all functional circuit boards are mutually connected in the manner of a system bus, the interface device, the relay switch matrix board and the A/D and D/A boards are electrically connected, and the detector and the CPU board are electrically connected. The invention has strong universality, flexible assembly, simple operation and high fault diagnosis ratio and is suitable for the online fault diagnosis and location of the circuit boards in various industries.