Abstract: We proposed a new method for measuring the solid linear expansion coefficient by alternating—current bridge(AC bridge)，which can be used to measure small capacitance.Converting solid tiny length change into AC bridge capacitance change.W e realized the accnrate measurement of solid linear expansion coefficient.W e made a rectangle plane—parallel capacitor as the capacitance of AC bridge whose bottom plate was supporting by testing metal bar.When the metal bar was heated，it expanded.So the gap of capacitor was minimized，and the capaeitance was changed.For reducing the temperature gradient of heating system，we added some experiment instruments of viscosity and temperature sensor into traditional steam jet heat meter.The experimental results showed that the method was simple，had high measuring accuracy and small error.

Key words: alternating-current bridge, capacitance;linear expansion coeficient;plane-parallel capacitor, temperature control system