A type of improved DNA genetic algorithm is proposed, which is used to solve the shortcomings like slow rate of convergence and easy precocity. The algorithm has increased the population diversity by using DNA encoding mode of the base complementarities, avoided falling into the local extremum. The replacement adaptive crossover operator and codon mutation operator are designed based on the DNA molecule operation, so it improves the genetic algorithm's search capability, and increases the convergence speed and efficiency. The algorithm is adopted to seek the best threshold of two-dimensional Arimoto. At last, the image is segmented. It is proved that this improved DNA genetic algorithm has improved segmentation validity by both the ordinary image and medical image.