We take an M_S4.7 earthquake that occurred at Zhongba, Tibet on January 4, 2017 as an example. The focal mechanism solution is inverted with the broadband waveform from a sparse network of Tibetan Regional Digital Seismic Network using the ISOLA full-waveform method. The best double-couple solution of the earthquake for two nodal planes is:strike=109°/dip=85°/Rake=-177° for Noodle 1 and strike=19°/dip=88°/Rake=-4° for Noodle 2. The centroid location and depth is 30.590°N, 83.784°E/6km;the moment magnitude is M_W=4.6, which shows a strike-slip faulting mechanism. The joint constraint inversion is conducted using the CSPS method with the P-wave polarity and local and near regional waveform data. The comparison of the results from two methods, including ISOLA and CSPS, shows that combining with the three-component waveform from a few stations can not only determine quantitatively the best solution but also avoid the non-uniqueness from the P-wave polarity inversion and constrain the instability due to the inversion with data from very few stations. A simple but effective way with high stability and reliability is provided in this paper to determine the focal mechanism solution of a small and moderate earthquake with the data from a sparse network.