The broadband tunable filter is considered as one of the key equipment to break the bottleneck of the information processing technology, which can effectively improve the transmission capacity and spectrum efficiency of information networks. An ultra-broadband tunable filter based on cascaded chirped subwavelength grating assisted contra-directional coupler is proposed. The filter is based on silicon on insulator. By using subwavelength grating and introducing chirp in the contra-directional coupler, an ultra-broadband filter is obtained. The bandwidth tunability of the proposed filter is realized by cascading contra-directional couplers. The transmission function of the contra-directional coupler is analyzed, and some feasible methods to enlarge the bandwidth of the filter are proposed. The ultra-broadband filter with a bandwidth of 64.07 nm, a sidelobe suppression ratio of more than 10.5 dB and insertion loss of about 0.60 dB can be realized by using a single contra-directional coupler. The ultra-broadband tunable filter with an insertion loss of 1 dB and a bandwidth tuning range of 44.3-49.92 nm can be realized by cascading contra-directional couplers. The ultra-broadband tunable filter has the advantages of large bandwidth, tunability, high similarity with rectangular shape, small size, and uninterrupted transmission, which can be widely used in optical filtering and information transmission.