An amplifier which has high speed and high precision was designed for monitoring the incident and reflected power of the electron cyclotron resonance heating system in magnetic confinement nuclear fusion. We propose a modular design technique and a reasonable process to design amplifiers with high speed and low noise. Some characteristics of the amplifier, such as stability, response speed, noise, offset, has been analyzed detailed. A labview based automatic amplifier test system was established. The response time and the precision of the amplifier have been tested and the results have been discussed. With proper configuration (Amplification factors of the three stage amplifiers are 10~20, 30~50, 2~20 respectively), the response time of the cascaded amplifier is less than 10 μs, the equivalent input noise voltage is less than 60 μV, and the equivalent input offset voltage is less than 32 μV even in the worst case. The characteristics, stability and reliability of the amplifier has been proved in Electronic Cyclotron Resonance Heating experiments.