Abstract: The pulsating flow and heat transfer characteristics of supercritical water in parallel water-wall tube are experimentally studied under variable load condition to avoid the serious effect of flow instability to water-wall tube. The material of the tubes is 1Cr18Ni9Ti. The size of the tube is Φ25×3 mm. The scope of experimental parameters is as follows: pressure from 23 to 30 MPa, mass flow rate from 0.05 to 0.2 kg/s; inlet water temperature from 200 to 390℃, inlet pressure drop coefficient from 0 to 5.5, and heat flux from 0 to 500 kW·m⁻². Different experimental groups are set up and the experiment is carried out by increasing the heat load of the experimental section. Different types of oscillations and their boundaries are observed through the test. The variation of mass flow rate and pressure in the channels are recorded. The oscillation curves of wall temperature are obtained. The influence of oscillation on heat transfer is analyzed. The results show low frequency, high frequency of type Ⅰ and high frequency of type Ⅱ appear successively with the increase of heat load. The low frequency pulsation and high frequency of type Ⅱ are in the same phase with the overall parameters. The high-frequency pulsation of type Ⅰ is the intertube oscillation. The increase of amplitude and frequency will increase the content of low-specific heat fluid in the channel for low-frequency oscillation. Moreover, the heat transfer capacity of the fluid will be reduced. For high-frequency oscillation, the increase of amplitude will weaken the boundary layer thermal resistance, which is conducive to heat transfer. The energy exchange between the fluids in the channel increases with the increase of oscillation frequency. The radial heat transfer between the tube and the fluid is inhibited. The study shows that the flow instability phenomenon has complex heat transfer characteristics. The operating conditions should be set outside the pulsating flow boundary.