Based on the experiment of trapping single cesium atoms in magneto-optical trap (MOT), a grating-external-cavity diode laser, which provides the cooling/trapping beams of MOT is locked to cesium 6S1/2 F=4→6P3/2 F′=5 cycling transition by modulation-free polarization spectroscopy. This frequency locking method uses the dispersion-like curve generated from polarization spectroscopy as frequency-discriminating signal. The error signal is feedback to both current modulation port of diode laser and voltage modulation port of piezoelectric transducer (PZT) in grating external cavity. The locking technique of modulation-free polarization spectroscopy can clearly improve the frequency stability compared with conventional saturation absorption spectroscopic technique. The minimum value of Allan variance is σy(τ) = 4.6×10-12 at average time of τ=300 s.