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Single-atom catalysts (SACs) have been widely used in Fenton-like water treatment, but studies on the selective induction of H 2 O 2 to produce singlet oxygen (1O 2) are rare. Herein, a carbon nitride supported high-loaded single-atom Cu-N 3 catalyst (Cu-CN, Cu load is 15.46 wt%) is prepared to activate H 2 O 2 to selectively form 1O 2. Experimental and DFT calculation results reveal that the key factor for 1O 2 production is the Cu-N 3 coordination structure. Specifically, Cu-N 3 coordination structure is conducive to decomposing H 2 O 2 into·OOH/·O 2 -. Besides, the density of Cu-N 3 sites is another key factor, high Cu-N 3 site density is conducive to the rapid conversion of·OOH/·O 2 - to 1O 2. Benefitting from the dominant role of 1O 2 , the Fenton-like degradation performance of Cu-CN/ H 2 O 2 system is not disturbed under high salinity conditions, and the performance is significantly enhanced at high pH. This work represents an important reference in understanding SACs for activated H 2 O 2 to generate 1O 2. [Display omitted] • Cu-N 3 single-atom catalyst (CuCN) can induce H 2 O 2 to selectively produce 1O 2. • The Cu-N 3 coordination structure is the key factor for 1O 2 production. • CuCN exhibits excellent Fenton-like performance under high salinity conditions. • The density of Cu-N 3 sites can modulate the Fenton-like active species. [ABSTRACT FROM AUTHOR]