Heterostructure has triggered a surge of interest due to its synergistic effects between two different layers,which contributes to desirable physical properties for extensive potential applications.Structurally stable borophene is becoming a promising candidate for constructing two-dimensional (2D) heterostructures,but it is rarely prepared by suitable synthesis conditions experimentally.Here,we demonstrate that a novel heterostructure composed of hydrogenated borophene and graphene can be prepared by heating the mixture of sodium borohydride and few-layered graphene followed by stepwise and in situ thermal decomposition of sodium borohydride under high-purity hydrogen as the carrier gas.The fabricated borophene-graphene heterostructure humidity sensor shows ultrahigh sensitivity,fast response,and long-time stability.The sensitivity of the fabricated borophene-based sensor is near 700 times higher than that of pristine graphene one at the relative humidity of 85％ RH.The sensitivity of the sensor is highest among all the reported chemiresistive sensors based on 2D materials.Besides,the performance of the borophene-graphene flexible sensor maintains good stability after bending,which shows that the borophene-based heterostructures can be applied in wearable electronics.The observed high performance can be ascribed to the well-established charge transfer mechanism upon H2O molecule adsorption.This study further promotes the fundamental studies of interfacial effects and interactions between boron-based 2D heterostructures and chemical species.