Sosemanuk is one of the stream ciphers that have been selected into the final portfolio for the eSTREAM project. Sosemanuk from the view point of bit-level was described. By studying the S-box of Serpent1, the modular addition, and the linear feedback shift register, a kind of equation between the bits of the internal states was obtained, based on which an improved word-oriented guess-and-determine attack on Sosemanuk is proposed. This improved attack utilizes the Groebner basis method to solve the equation system and can determine the other 5 words of the internal state by guessing just 7 words, and thus the complexity is pvoved to be O(2192).