The invention discloses a chromium catalyst for a nitrogen and phosphorous ligand framework and an application of the chromium catalyst in catalyzing ethylene oligomerization. The chromium catalyst for the nitrogen and phosphorous ligand framework is a ligand compound formed by chromium metal compounds and phosphorus and nitrogen organic ligands, substituent groups on the ligand framework, nitrogen and phosphorus are exchanged, selective trimerization and tetramerization of ethylene can be achieved, and meanwhile 1-hexene and 1-octylene are produced at the same time.