I figured out two ways:
1. In map viewer, you can manually set the visibility level of the tile layer to any scale regardless of the tile levels, so that the tile layer can be displayed in map viewer in all scales. However, if you use the map as a custom basemap, the Basemap Gallery widget in Web AppBuilder will not respect the visibility setting.
2. Modify the service properties in ArcGIS Server Administrator if you are using Enterprise. Change "maxScale" property to "70.5310735", and Basemap Gallery widget in Web AppBuilder will respect this setting.