We propose a spin filter based on both the quantum interference and the Rashba spin-orbit (RSO) effects. This spin filter consists of a Aharonov-Bohm (AB) interferometer with two quantum dots (QDs) inserted in its arms.The influences of a magnetic flux ψ threading through the AB ring and the RSO interaction inside the two QDs are taken into account by using the nonequilibrium Greens function technique. Due to the existence of the RSO interaction, the electrons flowing through different arms of the ring will acquire a spin-dependent phase factor in the linewidth matrix elements. This phase factor, combined with the influence of the magnetic flux, will induce a spin-dependent electron transport through the device. Moreover, we show that by tuning the magnetic flux,the RSO strength and the inter-dot tunnelling coupling strength, a pure spin-up or spin-down conductance can be obtained when a spin-unpolarized current is injected from the exteal leads, which can be used to filter the electron spin.