In order to overcome the defects such as low light emitting efficiency of double-capped cast light emitting diode (LED) daylight lamp using a general diffusion tube, a new diffusion tube with triangular microstructure as inner surface and with concave micro-lens microstructure as outer surface is studied. The principles of such a diffusion tube used for double-capped cast LED daylight lamp is introduced. A formula to calculate triangle microstructure′s base angles is derived. The influences of the parameters of the diffusion microstructure on the efficiency of the transmitted light are analyzed by simulations using TracePro software. The results show that, when triangle microstructure′s base angles are certain, triangular bottom width D has no effect on the light transmittance. Diameter d of the outer micro-lens surface has a greater impact on the transmittance, and the transmittance is the lowest when d=D/2. The shape of the outer micro-lens surface determines the divergence angle of light beam from this diffuser, and the divergence angle for a concave micro-lenses with cross-section being spline curve is greater than that with cross-section being perfect circular or elliptic curve. However，their transmittances are similar. The transmittance of this diffuser after optimization is about 0.92, the axial beam divergence angle is about 180°, and the radial beam divergence angle is 360°. The axial illumination uniformity of the daylight tube is better than 90% when using the improved double triangle inner surface microstructure.