For precision SG-Ⅱ power balance, there must have same output energy and output pulse waveform in the 8 beams, it means that the gain and transmissivity in every amplifier chain are same. But the gain abilities of the same type amplifiers in 8 beams are different for needing of daily operation. It was a first time that small signal gain of different amplifiers in SG-Ⅱ were measured online by using a small energy measurement system which has superior 1% precision. The small signal gain values of 40 and 70 rod amplifiers, 100 slab amplifiers and 200 double slab amplifiers are got in the regular working voltages. The differences of the same type amplifiers are quantized and that provides valuable experimental data to power balance in SG-Ⅱ precision.