Abstract: In order to estimate the sound intensity level of Vertical Transmit Array (VTA) from that of single transducer,a fast estimation method of Transmit Array Gain (TAG) in Pekeris waveguide is studied.At first,the average sound intensity of VTA is calculated on the basis of normal mode theory with effective depth,and the TAG expressed by the summation of modes is derived.Then by using integral instead of summation of modes order,the fast estimation expression of TAG in 3/2 power field is derived after error correction.Because only the first mode exists in Effective Shadow Zone (ESZ),the fast estimation expression of TAG in ESZ is derived by directly simplifying the former expression.The effective depth can be approximately replaced by the depth of water when the seafloor geoacoustic parameters are unknown.All of the fast estimation errors of computer simulations and experiment in shallow water are less than 1 dB,which can verify the validity of the fast estimation method.This fast estimation method of TAG can provide references for practical application.