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.