Based on the multibeam interference principle, a transmitting intensity expression of a Gaussian beam incidenting obliquely on an angletuned narrowband filter is derived. According to the result, the influence of the incidence angle on the transmission of a Gaussion beam is investigated theoretically. The calculation and experiment results show that the size of the transmitting beam spot is brodened obviously in oblique incidence. When the incident angle of the thinfilm filter is relatively small, the position of the peak of the transmitting beam is shifted and the beam is not in Gaussian distribution. When the incident angle of the filter is relatively large, the transmitting beam consists of a series of spaceseparated beams.