Abstract: The accurate nowcast and forecast of the ionosphere has important application value for the mitigation of equipment system effect and space science research. Ground-based GNSS and space-based occultation are one of the most important data sources for ionospheric data assimilation. The impacts of global ionospheric data assimilation under four different observation system conditions is quantitative analyzed by observation system simulation experiment (OSSE). The skill score(SKS) of total electron content (TEC) and three-dimensional electron density of data assimilation shows that: 1) Observation system has an important influence on the nowcast and forecast accuracy of data assimilation. 2) The assimilation TEC accuracy of ground-based GNSS data is significantly better than that of the background model. The SKS of TEC nowcast, 1 h, 2 h and 3 h forecast are 0.45, 0.39, 0.32 and 0.25 respectively. However, compared with the data assimilative results, the improvement of the background model is relatively limited, and the SKS of the nowcast, 1 h, 2 h and 3 h advance prediction of electron density is only 0.05, 0.04, 0.03 and 0.02. Through the fusion of occultation observation data, the accuracy of electron density will be improved obviously. 3) Under the condition of a large number of occultation satellites and good coverage of observation data, accurate TEC and electron density present and short-term prediction results can be obtained only by assimilating occultation TEC observation data. compared with ground-based GPS observation, the SKS of large-scale occultation constellation is increased by 0.2,0.17,0.14 and 0.12 respectively. The electron density SKS increases by 0.39,0.35,0.28 and 0.22 respectively, and the increase of electron density SKS is better than that of TEC; 4) The existing observation system layout improves the data assimilation accuracy in the middle and low latitudes better than that in the high latitudes. 5) With the increase of prediction time, the accuracy of data assimilation decreases. With the rapid increase of the number of microsatellites in the future, space-based occultation observations will play an inestimable role in the accurate characterization of ionospheric states, the related study can provide technical reference for the design and construction of space-based ionospheric observation system in China.