Take the Liujiaxia bridge as engineering background. Based on classical flutter theory, It focus on research critical factors which affect the critical velocity about the ultra-narrow steel truss stiffening girder suspension bridge in order to provide theoretical basis to the problem of low flutter stability of Liujiaxia bridge. The results show that critical velocity will decrease with increase about the vertical bending frequency and will increase with the torsion frequency and torsion-bend-frequency ratio. But the critical velocity will keep stabilize when torsion-bend-frequency ratio exceeds than 1.2. Though Changing sag-span ratio, width and height of beam can improve the critical velocity, it maybe increase cost of bridge remarkably when width and height of beam reach to some level. It is uneconomic if only adopting structural measures by means of optimization of design parameters, as a result, aerodynamic measures is more important.