This paper investigates the coupling transient behavior of primary frequency regulation of hydropower plant. Firstly, the model of hydropower plant with super long diversion tunnel under primary frequency regulation is established. Then, the stability is analyzed based on stable domain. The dynamic response is analyzed based on dynamic response process and indexes. The effects of system characteristics on stability and dynamic response are revealed. Finally, the coupling effect of head loss nonlinearity of super long diversion tunnel and water hammer elasticity of penstock on transient behavior is studied. The results indicate that the coupling effect of head loss nonlinearity of super long diversion tunnel and water hammer elasticity of penstock on transient behavior is influenced by sectional area of surge tank. With the decrease of sectional area of surge tank, the effect of water hammer elasticity of penstock is gradually weakened, while the effect of head loss nonlinearity of super long diversion tunnel is gradually strengthened. When the sectional area of surge tank is greater than critical stable value, the effect of water hammer elasticity of penstock plays a dominant role. The water hammer elasticity of penstock is unfavorable for the stability and dynamic response of primary frequency regulation.