Along with the rapid development of information technology, an increasing number of data need to be collected from the building electrical system. Therefore, it is critical to design a data acquisition system can be directly integrated with the existing building control system. In this paper, we concentrate on the research of fault diagnosis data collection. We design a MODBUS based data acquisition system to collect the fault signals involving current, voltage and harmonic, which can provide real-time and accurate fault data for the analysis and calculation in the building electrical fault diagnosis system.