A method and device for testing the thermal conductivity of a nanoscale material 1. The method comprises the following steps: preparing or placing a nanoscale material 1 to be tested on a substrate and plating an electrode 2 at both ends thereof; determining a resistance temperature coefficient R′ of the nanoscale material 1 to be tested and a resistance R0 at the ambient temperature T0; generating a small signal voltage V3ω with a frequency being 3ω on the nanoscale material 1 to be tested; and measuring the small signal voltage V3ω, and in conjunction with each piece of test data, calculating, according to a formula, the thermal conductivity κ of the nanoscale material to be tested 1 at the ambient temperature T0.