Based on the fractal theory and the idea of the Tree Growth Competition Algorithm(TGCA), a method of optimizing the resonant frequencies by iterating the branch structure at different growth factors is proposed, which overcomes the inconvenience in adjusting the position relationships of multi-band frequencies for common fractal antennas. The lower and upper resonant frequencies of the tree-growth fractal antenna can be optimized conveniently by linearly changing its size and growth factors. Based on this method, a tree-growth fractal antenna is optimized in the GSM900/DCS1800 frequency band by using Finite-Difference Time-Domain(FDTD) algorithm. The measured and simulated results indicate that the bandwidths of microwave fractal antenna are both above 100 MHz at the resonant frequencies of 0.91 GHz and 1.81 GHz with good omni-directional radiation patterns in the horizontal direction. These good agreements of the measured and simulated results validate the feasibility of the proposed method.