The invention relates to a monitoring system of capacity features of overhead power transmission wires, a test method and a device. The monitoring system comprises a microclimate test device, a system main station and multiple groups of test loops erected between power transmission line towers. Each test loop comprises two to-be-tested wires connecting a heavy current generator, a data acquisition device arranged in each to-be-tested wire and a current test device arranged on any to-be-tested wire. According to the invention, sag features and different wires and migration point temperatures in multiple groups of test loops can be researched and compared; controllable heavy current is generated through the heavy current generator and the current test devices, so precise control on wire temperature is achieved; temperature sag relation curves of wires can be acquired under the current which is changed in a wide range; the migration point temperatures of the wire are further precisely determined; and the monitoring system, the test method and the device are suitable for monitoring of capacity features of all kinds of overhead lines.