To accommodate a low cost, portable and intelligent requirement for test devices of electrical equipments, this paper proposes an adaptive technique for optic IRIG-B codes and FT3 sample value codes. Based on the different structure properties and bit rate between IRIG-B codes and FT3 codes, the technique can automatically recognize those two kinds of codes by processing the input stream through a serial port with high frequency clock signal. This technique extends the functions of test device for smart substation. The handheld test device using this technique has been successfully applied in the system test and field test for several smart substations. The test results demonstrate the validity and practicability of the proposed technique, which reduces the number of ports of test device with decreasing its power consumption and cost.