Ni-based single crystal superalloys have been widely applied to turbine blades of aero-engines due to improved mechanical properties at high-temperature environments. There were great progresses on alloy design and sixth-generation Ni-based single crystal superalloys have been developed in the last decades. However, contents of refractory elements such as Re and Ru increase consistently. Additions of refractory elements lead to increase of cost and density of superalloys, which is not beneficial to massive application of alloys. Mo and W are potent solid-solution strengthener with lower cost, and they could serve as main strengthener in single crystal superalloys with low densities and costs. The effects of Mo and W on single crystal superalloys have been widely researched. The present work reviewed research from distribution of elements, microstructure of γ phase and γ′ phase, microstructural stability, creep properties, solidification defects and oxidation and hot corrosion resistance at high temperatures. Finally, we analyzed the current shortcomings of related research and pointed out the potential direction of future research.