Objective: To investigate the alterations in surface electromyography (sEMG) of the tibialis anterior and gastrocnemius muscles during straight-line walking in Parkinson's disease (PD) with freezing of gait (FOG), and their correlations with clinical features. Methods: Twelve PD patients with FOG, 13 PD patients without FOG, and 11 healthy controls (HCs) underwent clinical assessments, gait kinematics acquisition, and sEMG evaluations during straight-line walking. The sEMG signal characteristics of the severely affected tibialis anterior and medial head of the gastrocnemius muscles during different phases of the gait cycle were analyzed using the normalized root mean square (RMS) and co-activation ratio. Additionally, the correlations between sEMG alterations and clinical features were explored. Results: In comparison with HCs and PD patients without FOG, PD patients with FOG demonstrated notable deceleration in gait speed, reduction in stride length, diminished swing phase, and increased gait variability (p < 0.05). During the single support phase, FOG patients exhibited decreased normalized RMS of the tibialis anterior compared to HCs (p < 0.05). In the pre-swing phase, the normalized RMS of the tibialis anterior was significantly lower in PD patients with FOG than those without FOG (p < 0.01), while PD patients without FOG showed elevated normalized RMS of the tibialis anterior compared to HCs (p < 0.01). Furthermore, the normalized RMS of the gastrocnemius muscle decreased during the pre-swing phase for FOG patients when compared to those without FOG and HCs (p < 0.05). Additionally, the co-activation ratio of the tibialis anterior and gastrocnemius muscles was reduced during the swing phase in FOG patients compared to patients without FOG (p < 0.05). Finally, significant correlations were identified between the normalized RMS of the gastrocnemius muscle during the pre-swing phase and FOG severity (r = -0.758, p = 0.007), co-activation ratio during the swing phase and stride length variability (r = 0.716, p = 0.013) in PD individuals with FOG. Conclusion: The decreased sEMG activity of the tibialis anterior and gastrocnemius muscles during the pre-swing phase, as well as the reduced co-activation ratio during the swing phase, represented significant characteristics in PD patients with FOG. These findings contributed to a deeper understanding of the pathophysiological mechanisms underlying freezing of gait.