Reasonable design of heterojunction can greatly improve the photocatalytic hydrogen evolution activity of materials. Herein, p-n heterojunction of 2D/3D structure is constructed by the nanosheet of CoAl-LDH and rock-like CuI. The introduction of CuI can make CoAl-LDH disperse better, which brings more reaction sites for the hydrogen evolution reaction. Meanwhile, the 2D/3D structure is conducive to the construction of p-n heterojunction between the CoAl-LDH and CuI. The optical and electrochemical properties of the material indicate that the separation and transference of photon-generated carriers are promoted by the p-n heterojunction. The activity of composite catalyst (CI-10) reaches a maximum of 3.59 mmol g^-1h^-1 which is 28.5 times higher than that of CuI. Furthermore, the influence of the amount of CuI and pH value on the hydrogen evolution reaction is explored. Based on the band structures of CoAl-LDH and CuI, the mechanism of photocatalytic reaction of CI-10 is proposed. The p-n heterojunction constructed with the CuI as hole receptor provides a new way to enhance the activity of photocatalytic H₂ evolution.