The red-shifted second harmonic emission of the laser light is observed in the laser propagation direction on the rear side of a foil target irradiated by femtosecond laser pulses. The emission is attributed to the coherent transition radiation (CTR) when the hot electrons accelerated by the v×B heating process traverse the solid-vacuum boundary. The red shift is caused by the expansion of the critical density surface towards the vacuum side. With the increase of the prepulse energy, the peak of the CTR moves further to the long wavelength side causing the broadening of the spectrum. The measurements of the CTR provide a potential diagnosis to the movement of the critical surface in the laser-plasma interaction process.