Disclosed are an online alternating current detection device and method. The method includes the following steps: firstly fixedly connecting two sampling points to a segment of wire; amplifying the voltage difference of the sampling points to obtain a median value; subsequently rectifying and filtering and amplifying the median to obtain a direct current amplified value; then utilizing the amplified value, the median value, the multiplying power there between and the resistance value of the wire between the two sampling points to calculate an estimated value of an alternating current; comparing the estimated value to a reference value measured by a standard device between the two sampling points; then adjusting the multiplying power of the amplified value relative to the median until the estimated value coincides with the reference value, only then can the alternating current be measured online. The present solution employs a microvolt voltage measurement manner, there is no need to perform sampling processing in auxiliary components besides an online wire, so there is no need to consider the measuring range, power consumption, cost and specification problems of the auxiliary components, and the performance is stable with good consistency. At the same time, the security is good and the measuring range is wide.