Based on the diffusion Karma phase field model with thin interface limit, the dendrite growth behavior of pure Ni undercooled melt was simulated under different temperature coupling strength. Results show that as the thermal coupling strength decreases, the impact of the temperature field on the phase field weakens, the solid-liquid interface disturbance increases, and secondary dendrites develop gradually from the main branch. Meanwhile, the dimensionless growth velocity of dendrite tip increases, while the dimensionless dendrite tip radius decreases. Furthermore, at the initial solidification stage, the transient growth velocity of dendrite tip is greater than the theoretical stable growth velocity; with the growth process going on, the growth velocity of dendrite tip gradually decreases until it equals to the theoretical stable value. Therefore, the computational results by the phase-field method are in good agreement with that by the microsolvability theory.