The thermal conductivity of the straw ash concrete with the compressive strength of C20 was tested dependent on the sand ratio, the water cement ratio and the straw ash content by using steady state plate heat conduction tester. The results showed that there was a negative correlation between the thermal conductivity of ash concrete and the straw ash content. When the rape straw was added, the thermal conductivity of the straw ash concrete was decreased by about 6.8% ~ 21.0% compared with the concrete block without straw ash. The thermal insulation performance of the straw ash concrete was improved with an increase in the rape straw ash content. When the water cement ratio is 0.52, the straw ash content is 5%, and the sand ratio is 30%, the straw ash concrete possessed the best heat insulation performance with the compressive strength of 22.8 MPa.