We use nonlinear polarization rotation technology to realize the self-start passive mode-locking in the erbium-doped fiber ring laser of positive dispersion, which can generate a new kind of gain-guided soliton pulse. The laser generates picosecond pulses with a rectangular-shaped spectrum, the FWHM width of which can be greater than 19 nm. The gain-guided soliton formation mechanism may include laser gain, spectral filtering effect, cavity dispersion and other fiber nonlinear effects. In this paper we also study the chirp property, peak power and other parameters of the pulse. With further increasing the pump power，the laser can operate on dual-pulse output mode.