In this paper, a new phase retrieval method using sequential phase modulations is proposed. Behind the unknown object, adding sequential phase modulations will change the diffraction intensities received by sensor. Through increasing the number of diffraction intensities patterns, the difficulty of retrieving the unknown object is decreased. To better select these modulation phases, the complexity parameter is defined to evaluate the complexity of unknown object. When the complexity parameter of unknown object is larger, it contains more spectrum information on different frequency bands and will be harder to retrieve. The complexity of unknown object should be contained between the maximum and minimum complexities of modulation phases. In this way, the information of each frequency band on the unknown object can be effectively retrieved. Meanwhile, the distribution of modulation phases should be continuous to avoid introducing high frequency noise. In addition, there is no limit on what kind of modulation phase distribution to choose. The effectiveness and fast convergence of this new method has been proved.