The combustion-gas generator and cylindrical stepped-wall observation chambers with five stages are designed to study the expansion characteristic of twin combustion-gas jets in liquid working medium under high temperature and high pressure. The expansion processes of Taylor cavities formed by combustion-gas jets and the mixing characteristics of gas-liquid are studied by means of high-speed digital camera system. The effects of the distance between the two nozzle holes, injection pressure and nozzle diameter on jet expansion processes are discussed. The experimental results indicate that, the velocity differences exist on the gas-liquid interface during expansion processes of twin combustion-gas jets, and the effect of Taylor-Helmholtz instability is intense, so interfaces between gas and liquid show turbulent folds and randomness. The strong turbulent mixing of gas and liquid leads to release of combustion-gas energy with the temperature decreasing. Moreover, the mixing effectiveness is obviously enhanced on the corners of each step of the cylindrical stepped-wall structure, forming radial expansion phenomenon. The reasonable matching of multi-parameter can restrain the jet instability and make the combustion-gas energy orderly release. Based on the experiments, the three-dimensional unsteady mathematical model of interaction of twin combustion-gas jets and liquid working medium is established to obtain the density, pressure, velocity and temperature distribution characteristics in jet flow field. Discussions are focused on variation law of characteristic parameters of the flow field under different nozzle intervals. The results indicate that, as the ratio of diameter increment of each step to the nozzle interval Delta D/S >= 0.6, twin combustion-gas jets begin converging earlier, and the jets can also reach the exit of observation chamber earlier. But as Delta D/S < 0.6, the twin jets are closer to the chamber wall, and radial inductions from the expanding steps are stronger, so twin jets do not appear the collection effect on the space and time scale of the observation chamber and the pressure distributions of jet flow field are always more violent pulsation state. The calculated data of axial displacement of Taylor cavity heads are in good agreement with the experimental ones according to dealing with the density profiles and experimental sequence processes. (C) 2014 Elsevier Ltd. All rights reserved.