It is observed that the pulse duration of loosely focused pulses is self-compressed during the nonlinear propagation in argon when the input pulse peak power is close to the threshold power for self-focusing, and the results are confirmed by the numerical simulation. From the simulation we find that, near the lens focus the central part of the beam moves forward to the outer part owing to the plasma generation, and produces a leading peak in the temporal profile. And then, with the decrease of the plasma density, the spatio-temporal focusing and self-steepening effects predominate and promote a shock beam structure with a steep trailing edge. It is also found that, for our calculation case with the input pulse power close to the critical value of self-focusing, group velocity dispersion and multiphoton absorption effect have little influence on the propagation process, but the spatio-temporal focusing and self-steepening effects play a significant role in promoting the final pulse shortening.