In this paper, a low noise preamplifier for 0.5T small joint MRI instrument is designed,a feasible scheme of LNA production in low field MRI instrument is presented. Amplifier using two stage structure,the first stage uses a high electron mobility transistor ATF54143, the second stage uses a bipolar Darlington transistor ADA4743. The performance of the amplifier is good according to the testing of finished goods,and achieves the predetermined requirements. The working band of the amplifier is 18~23 MHz, the noise coefficient of center frequency is about 0.6 dB,the gain is about 40 dB, fluctuation in frequency band is small than 1 dB,and Input VSWR and output VSWR are all small than 1.5.