The fluidity of superalloy is an important property in the manufacture of large thin-walled and complex structure castings, but superalloys show high viscosity and a wide solidification temperature range, which may lead to serious defects, low yield, and poor performance. Therefore, the manufacturing of these castings is important to improve the fluidity by regulating the alloy composition. B and Zr, as common elements in superalloys, have important effects on solidification, microstructure, and mechanical properties. However, the effect of B and Zr on the fluidity and mechanism of superalloys remain unclear. In this study, the spiral fluidity test, high-temperature confocal laser scanning microscopy, and double-thermocouple method were used to investigate the fluidity, solidification, dendrite coherency point temperature, and stress rupture property of the IN718 superalloy with various B and Zr contents, whereas the mechanism of the effect of B and Zr on fluidity was also discussed. Results show that the combined addition of B and Zr can improve the fluidity and high-temperature stress rupture property of the IN718 superalloy. At different pouring temperatures, the fluidity of the alloy increases with the increase of B and Zr contents. The optimum fluidity is obtained with 0.0059%B and 0.042%Zr (mass fraction), respectively, which is about 90% higher than that of the original alloy and increases the stress rupture life by 77%. In the IN718 superalloy with a high content of B and Zr, the dendrite growth rate in the mushy zone is low, which reduces the dendrite coherency point temperature (T_DCP), increases the difference between liquidus temperature (T_L) and T_DCP (T_L- T_DCP), delays the dendrite coherency, and improves the melt fluidity.