The present invention provides a transformer blocking controller and a data collection unit error correction method. A data collection unit comprises a PT mutual inductor for voltage sampling and a CT mutual inductor for current sampling, if the transformer blocking controller is a blocking mode, the direct current of a transformer neutral point is detected, if the detected direct current is not zero, it is determined that there is a null shift current error, and the null shift current error value is employed to perform direct correction of the CT sampling value in the grounding mode; if the transformer blocking controller is a direct grounding mode, the voltages of two ends of a blocking capacitor are detected, if the voltages of two ends of the blocking capacitor are not zero, it is determined that there is a null shift voltage error, and the null shift voltage error value is employed to perform correction of the PT sampling value in the blocking mode. The error correction method does not need human intervention, can eliminate errors of the current mutual inductor itself, the voltage mutual inductor itself, the line and other devices, improve the sampling precision of current or voltage and allow the blocking device to more stably and reliably operate.