In order to confirm the effect of Mg and Al powders on combustion performance and boron oxidation efficiency of boron-based fuel-rich propellant in gas generator, the burning rate of three types of propellants in different batches were studied at various pressures and the contents of boron trioxide (B₂O₃) and total boron (B) in propellant combustion residues were determined by chemical method. The experimental results show that Mg content in propellant affects the combustion performances of propellant obviously. When the content of boron and total contents of Mg and Al in propellant are 30% and 6%, respectively, it was designed that the different ratios of Mg and Al in the propellant were as follows: the content of Mg is 0%, 3%, 6%, whereas, the content of Al corresponds to 6%, 3%, 0%, respectively. Burning rate and pressure exponent of the propellant increase with increasing of Mg powder content. The contents of B₂O₃ in combustion residues are related to Mg content in propellant. When Mg content in propellant increase, the content of B₂O₃ in combustion residues decrease; the efficiency of oxidized boron decrease with increasing of Mg content in the propellant and increasing of the combustion chamber pressure. Mg powder in the propellant can restrain the oxidation reaction of boron in the condensational state of the propellant and can increase the burning rate and pressure exponent for the boron-based fuel-rich propellant.