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To explore the ionospheric coupling mechanism, the total electron contents (TECs) and slant TECs (STECs) were processed from GPS sites in China, Nepal and India, and the singular spectrum analysis (SSA) method was introduced to investigate the seismic-ionospheric signals prior to the Mw7.8 Nepal earthquake on April 25, 2015. The results of pre-earthquake ionospheric anomaly show that there were obviously positive on 2nd day and negative anomalies on 14th day prior to the earthquake, respectively. The radius of anomalous area with a regular elliptical shape reached 2500 km and 2000 km in the above days respectively, which was close to the radius of Nepal earthquake. After excluding the moving effects of ionosphere hump, the investigation by the global ionosphere maps (GIMs) shows that a wide TEC abnormality appeared over the epicenter and conjugated region with the duration of 12 hr. Besides, the magnitude of TEC anomalies impending the shock, with its peaks happening at the midday (local time), was proportional to the number of days prior to the occurrence. By using the STECs within seismogenic zone, we further studied the coseismic ionospheric disturbance (CID) of Nepal earthquake. The results show that two modes of CID propagation velocity have been detected. Due to the TEC respond of Rayleigh wave, one mode had average propagating speed of 1.3 km/s in the near-field within 800 km, and gradually reaching up to 2.7 km/s in the far-field beyond 2000 km. Another mode had average propagating speed of 0.6 km/s only existing in the near-field, which was in accordance with the speed of shock acoustic waves. Besides, CID intensity presented the obvious directional difference. The ambient magnetic field around the epicenter made it easier for CID to spread in the southward direction, the plateau topography also prohibited the propagation of ionospheric disturbance, and most CID points were detected along the southeast direction of the epicenter which is consistent with the rupture propagation. [ABSTRACT FROM AUTHOR]