The emission nebula is very common in interstellar matter, and its continuum has several obvious emission lines. In order to study the chemical elements in the emission nebula, this paper uses a small optical telescope with Hα, [SII], [OIII] narrow-band filters to select four emission nebulae for observation. A lot of results can be obtained by analyzing the experimental data. For example, in most of the emission nebulae, the content of Hα is much larger than [SII] and [OIII], and in the supernova remnants, the content of Hα is less. This study proposes a new method of extended source photometry, and estimates the brightness of the selected emission nebula in each band. This method also helps to plan the exposure time for large telescopes, thus can improve observation efficiency.