Hollowcore photonic bandgap fiber (HCPBF) has excellent environment adaptability, but highorder modes (HOMs) in HCPBF limit its applications in many areas. A method for singlemode propagation is proposed, which is based on the phenomenon that the HOMs are seriously suppressed and the fundamental modes stably exist when the operating wavelength is less than the cutoff wavelength. The method is experimentally verified. The influence of HCPBF cladding structure parameters on the bandwidth and operating wavelength for singlemode operation is simulated. The results show that the singlemode bandwidth is broadened by ~2.9 times when the index increases from 1.445 to 2, and singlemode operating wavelength can move ~310nm when the ratio between fillet diameter and hole diameter increases from 0.2 to 0.8.