  The hybrid Tibet AS array was successfully constructed in 2014. It has 4500
m$^{2}$ underground water Cherenkov pools used as the muon detector (MD) and
789 scintillator detectors covering 36900 m$^{2}$ as the surface array. At 100
TeV, cosmic-ray background events can be rejected by approximately 99.99\%,
according to the full Monte Carlo (MC) simulation for $\gamma$-ray
observations. In order to use the muon detector efficiently, we propose to
extend the surface array area to 72900 m$^{2}$ by adding 120 scintillator
detectors around the current array to increase the effective detection area. A
new prototype scintillator detector is developed via optimizing the detector
geometry and its optical surface, by selecting the reflective material and
adopting dynode readout. This detector can meet our physics requirements with a
positional non-uniformity of the output charge within 10\% (with reference to
the center of the scintillator), time resolution FWHM of $\sim$2.2 ns, and
dynamic range from 1 to 500 minimum ionization particles.
