Organic　semiconductors　show　great　potentials　for　applications　in　photocatalysis　to　provide　solutions　for　environmental　remediation　and　energy　harvesting.　Herein,　we　report　the　preparation　and　photocatalytic　characteristics　of　a　small-molecule　organic　semiconductor　of　pyromellitic　diimide　(PD).　The　one-pot　heat-induced　imidization　reaction　of　pyromellitic　dianhydride　and　2,4,6-triaminopyrimidine　obtained　PDs　with　different　phase　and　crystal　structures.　The　crystalline　grains　of　PD　(c-PD),　which　is　composed　of　layered　constituents　with　pi-pi　stacking　structure,　effectively　photodegraded　a　series　of　dyes　including　methyl　blue,　methyl　orange,　orange　II　sodium　salt　and　sulforhodamine　B　under　UV　illumination.　Moreover,　c-PD　is　featured　with　dual　selective　photodegradation　behaviors　in　favor　of　both　azo　dyes　and　cationic　dyes　in　their　respective　mixtures　with　non-azo　dyes　and　anionic　dyes,　which　has　not　been　reported　before.　The　preferential　photodegradation　of　the　azo　dyes　might　relate　to　their　enhanced　pi-pi　interactions　with　c-PD.　In　comparison,　the　selective　photodegradation　of　the　cationic　dyes　is　ascribed　to　preferential　adsorption　of　the　dyes　due　to　electrostatic　attractions　between　opposite　charges.　The　work　provides　new　approaches　for　developing　advanced　photocatalysts　with　selective　photocatalytic　behaviors　for　controlled　treatment　of　waste　water　by　photocatalysis.