A data transmission detecting device including a detecting module and a detection value calculating module is provided. The detecting module has a plurality of receiving terminals and receives a first data and a second data during a first period. The detecting module calculates a total detection value according to the first data and the second data, and performs an error check comparison by comparing the total detection value with an error check code. When the detecting module again receives the first data during a second period, the detection value calculating module transmits an auxiliary detection value to the detecting module, so that the detecting module calculates a corresponding total detection value according to the auxiliary detection value, and performs the error check comparison by comparing the total detection value with the error check code. The first period and the second period are two successive periods adjacent to each other.