Using the scalar method, the field distribution of the linear polarization in a step-index of weakly guiding and the hegeneral expression of the mixed modes are derived. The results show that, the mode field ranges of the lowed LPmn increases with the fiber core radius and the coating layer index. The mode field ranges of the lowest order LPmn modes reduces with the increase of the fiber core index. The optical strength of the mixed modes reduces with the increase of the transmission distance and the proportion of mixed modes. The M2 factor of the uncoherent mixed modes increases gradually with the increase of the proportion of the highest order LPmn modes. Howerer, the increase of M2 factor approaches a critical value when the fiber core radius is a=30 μm.