One of the solutions of water-alternating gas injection technology and utilizing of associated petroleum gas enhancement is application of pump-ejector system technology, which uses a liquid-jet gas pump (LJGP) as a water and gas mixer. This paper is devoted to the test bench studies of the LJGP's performance characteristics with change of the mixing throat's length at excess pressures of the ejected gas. As a result of the bench studies, it was obtained that the excess pressures of the ejected gas Pin require a different approach when choosing the optimum length of the LJGP's mixing throat, as compared to atmospheric pressure. So, it was determined that at Pin from 0.05 to 0.18 MPa, a long mixing chamber should be chosen, and Pin from 0.18 to 0.6 MPa, the optimal length is medium. It was also obtained that with an increase in Pin, the optimum working range of the mixing throat lengths increases, which makes it possible to select the mixing throat in wider ranges, without losing the efficiency of the LJGP. With the optimal length of the mixing throat, the highest gas injection coefficients were in the range of Pin = 0.2-0.5 MPa. © Published under licence by IOP Publishing Ltd.