The invention relates to a two-slit interference and single slit diffraction experimental instrument which comprises a horizontal plate. The two-slit interference and single slit diffraction experimental instrument is characterized in that a supporting rod is fixedly arranged on one side of the middle of a first supporting column, a laser is horizontally and fixedly arranged at the top end of thesupporting rod, a first sleeve is horizontally and fixedly arranged at the top end of the first supporting column, the laser and the first sleeve are located on a same horizontal line, a second sleeveis arranged out of the end, far away from the laser, of the first sleeve in a sleeving manner, a gap head is arranged at the end, far away from the first sleeve, of the second sleeve in a sleeving manner, and sliding blocks are fixedly arranged at the bottoms of a second supporting column and a third supporting column. By arranging the adjusting supporting columns and a horizontal tester, the horizontal degree of the horizontal plate is adjusted conveniently, so that the experimental result is more accurate. By arranging a brightness adjustor, the brightness of a light source sent by the laser can be adjusted. By arranging a first fastening screw, the gap head is replaced and mounted conveniently. By arranging the sliding blocks, the positions of a light screen and the gap head are adjusted conveniently.