One of the key questions in algebraic attacks is how to effectively reduce the degree of the function. Based on the algebraic attack, this paper presents a new algorithm of attack - chosen IV algebraic attack, which can reduce the function¿s degree by choosing appropriate IV. Using this algorithm, we analyze the One.Fivium, the predigesting form of Trivium. we can get 70 key bits, and the length of the longest key stream needed can be reduced to halve the length of the previous one. In order to fit the original intention, when the setup clock cycles in the key generating stream are increased to 630, we can still get 55 key bits by increasing the changeable bits of IV.