In total reflection prism laser gyro, the combiner output characteristics directly affect the gyroscope′s accuracy. By using the laser mode theory, the combination spot patterns of clockwise and anti-clockwise beams in different rotation speeds are analyzed. The interference spot abnormalities caused by unsuitably assembled diaphragm or combining prism, passed higher-order transverse mode, too big y direction angle, as well as multi-longitudinal modes are discussed. The relationship between beam combination quality and navigation accuracy in ring laser is studied experimentally.