In order to solve the reflection of the germanium surface,the square pillar subwavelength structures are analyzed and designed by model theory. The results show that filling factors that are bigger than 0.78 have a more important effect on transmittance when periods is smaller. Zero transmittance decreases so quickly with periods increasing while sum transmittance decreases so few with periods increasing when periods are bigger than 0.25λ. The relationships between transmittance and depth are cosine when the period is smaller while the relationships between transmittance and depth are pseudo cosine when period is bigger. Besides transmittance is high within wide incident angles when the period is smaller,when the period is bigger the subwavelength structures can be made narrowband filter.