Polarization is one of the basic properties of electromagnetic waves and is valuable in communication, navigation and radar detecting. So it is important to control and manipulate polarization states of electromagnetic waves. In this paper, we design, fabricate and measure a broadband reflective metamaterial 90 degrees polarization rotator which has a double-split-ring resonator (DSRR) structure, composed of two layers of dielectric and a metal plate ground. The explanation of the physical mechanism of the polarization rotator is presented according to the anisotropy media theory. Anisotropic metamaterials can cause a phase or amplitude difference between two crossed polarization waves, which can be used to manipulate the polarization states of the incident waves. The anisotropic polarization rotator behaves different for two orthogonal axes, and the surface current distributions of the DSRR are discussed to analyse the different characteristics of the structure along two orthogonal axes. The DSRR behaves as a dipole resonator that couples with the electric component along one axes and behaves as an LC resonance circuit that couples with the other electric component. Thus, almost an equal magnitude and a 180 degrees phase difference can be generated between the two orthogonal electric components of the reflected waves. The polarization states of the reflected waves will be rotated by 90 degrees, when incident waves are polarized by 45 degrees with respect to the symmetric axis of the rotator, and it will be retained when the incident waves are circularly polarized. Simulation results show that this device can work with the relative bandwidth of 90% from 5.5 to 14.5 GHz, of which the polarization conversion ratio is larger than 90%. The polarization conversion ratio will decrease as the incident angle increases, but this high polarization conversion ratio can be obtained at several frequencies. A 576-cell (24 x 24) prototype of the polarization rotator has been fabricated using a printed circuit board method on the FR4 substrates and the experimental results agree well with that of the simulation. The polarization rotator has a simple geometry but more operating frequency bands, compared with the previous designs. It provides a route to broadband polarization rotation and has application values in polarization control, design of new antenna and stealth technology.