In order to improve beat frequency stability in the dual-frequency laser with piezoelectric transducer for frequency stability, various error factors were analyzed, and with special attention on mode pulling effect. Due to mode pulling effect, beat frequency stability was influenced by path length, gain and temperature. When an ordinary power supply was used, the beat frequency stability of the laser was 4.5×10-8 in a constant temperature room. When a high-stability power supply was used, the beat frequency stability was 1.2×10-9. Beat frequency has relation between current and temperature was obtained, which also showed gain and temperature had important influence on beat frequency. Therefore, in order to obtain high beat-frequency stability in the dual-frequency lasers, variations of path length, current and temperature should be reduced.