A switchable operation of rectangular dissipative soliton and Gaussian-spectrum pulse has been investigated experimentally in the mode-locked fiber laser using single-walled carbon nanotube and nonlinear polarization rotation technique. The rectangular dissipative soliton is strong chirped with the temporal and spectral width of ~6.33 ps and ~12.54 nm, respectively. By appropriately adjusting the polarization states, the Gaussian-spectrum state with ~1.87 ps duration and ~2.2 nm bandwidth is achieved. The time-bandwidth product of the Gaussian-spectrum pulse is 0.51, suggesting that the pulse is nearly chirp-free. The switching of different operations could be ascribed to the change of cavity polarizations relying on polarization controllers, which is also demonstrated by the simulation results.