A laser interferometer to measure gravitational acceleration is built from low cost college laboratory apparatus. The device is basically a Michelson interferometer，in which the movable mirror is placed horizontally on the flat end of the plunger of an electromagnet. When the electromagnet coil is energized，the plunger will be quickly sucked back，leaving the movable mirror fall freely in air. At the stage of falling，the interference light intensity is recorded by an oscilloscope，and by fitting the signal with local sine waves，position of the movable mirror can be obtained at a precision much higher than the half wavelength of the laser. Finally，gravitational acceleration is extracted from the displacement - time curve of the movable mirror by a quadratic fitting. By using the data of 10 ms falling，which corresponds to displacement about 0. 5 mm，repeatable result of the gravitational acceleration in Beijing is obtained as 9. 803 ± 0. 003 m /s2 .