This paper proposes to divide the globe into meshes and select one GNSS receiver from each mesh. With the observation of these receivers, it develops a TEC derivation algorithm based on the spherical harmonic function. For different mesh sizes from 2.5°×2.5° (latitude×longitude) to 30°×30° (latitude×longitude), the performance is examined by applying it to the data selected from 16 geomagnetically quiet days in four seasons in 2006. The result shows that the data can be generally well fitted, with the goodness of fit being about 2 TECU. By comparing the derived TEC and instrumental biases with those of CODE (Center for Orbit Determination in Europe), it can be found that the algorithm can give reasonable TEC and biases for those meshes smaller than 15°×30°(latitude×longitude). Considering the Ionospheric puncture point distribution and calculating time, it can be concluded that the mesh of 10°×20° (latitude×longitude) is appropriate to derive TEC and instrumental biases rapidly. This paper provides a fast and effective method for nowcasting the global ionospheric TEC.