The instability of supercritical helium flow is investigated experimentally, Three types of instabillity are observed and their mechanisms are revealed. A type of instability is considered to be a Helmholtz one, caused by the acoustic resonance in the outlet pipe from the cryostat outlet to the flow control valve. The other two types are found to be the density wave instabilities caused by the density wave resonance, one in the entire system and another in the heated section.