A safety belt buffering device (14) and a related vehicle safety seat (10) are provided. The safety belt buffering device (14) is applied to a shoulder belt (16). The safety belt buffering device (14) includes a buffering component (28), a blocking component (30) and an actuating component (32). The buffering component (28) includes an accommodating space (34), and the accommodating space (34) is formed through the buffering component (28). The blocking component (30) is disposed inside the accommodating space (34). The actuating component (32) is disposed on the shoulder belt (16). The shoulder belt (16) movably passes through the accommodating space (34). The actuating component (32) is driven to move into the accommodating space (34) to contact the blocking component (30) when the shoulder belt (16) is pulled by external impact, and the blocking component (30) is structurally deformed to reduce the external impact. The present invention can obviously decrease the impact energy transmitted from the vehicle safety seat (10) to the passenger, so as to protect safety of the passenger.