The progress of the Er:glass lasers with passive Q-switch is discussed. The analytical expressions of the output energy and pulse width are derived. The energy, pulse width and peak power of the output pulse as functions of the cavity loss are numerically simulated. The relation between the output pulse energy and input pumping energy is numerically simulated. An Er:glass laser with LaMgAl11O19:Co2+ passive Q-switch was built to verify the numerical results. The results show that the increase of cavity loss results in widening of pulse width, decling of pulse energy and peak power of the output pulse. There is only single-pulse output when the input energy is less than 12 J. When the energy is over 12 J, it brings double-pulse output. 1.535 μm laser output with energy of 2.7 mJ and peak power of 50 kW was gained in the experiments by the power supply of 8 J. In the end, the methods to increase the output pulse energy are discussed.