Based on the multi-beam interference principle and Gaussian beam transmission equation in the free space, the intensity expression of a Gaussian beam incidenting obliquely on an angle-tuned narrowband filter has been derived. According to the result, the influence and relationship of the incident angle on the reflection of a Gaussian beam have been investigated theoretically. The calculation and experiment results show that the size of the transmitting beam spot will be brodened obviously when the incident angle is increasing. However, the size of reflecting beam spot will be a little brodened for a relatively large incident angle, which extent is much less than that of the transmitting beam. The reflecting port of the angle-tuned narrowband filter does not need facular-shaping fitting as the transmitting port within its tuanble range.