In end-pumped Nd:YVO₄ amplifiers, beam quality improvement is obtained both in theory and in experiments. A theoretical model of gain-guided laser amplifier is developed by comprehensively considering thermal effect, gain guiding and gain saturation effect. Several key parameters of the amplifier are discussed such as the input beam quality, the beam filling factor between input beam and pump beam, the ratio between input power and pump power, and the length of laser crystal. The theoretical results are confirmed by the experiments.