The invention belongs to a Co-bis-imidazolium metal framework compound and its preparation method and application; the chemical formula of the metal framework compound is: {Co2(1,2-BIB)(BTC)}n, wherein, 1,2-BIB It is 1,2-methyleneimidazolylbenzene, and BTC is deprotonated 1,2,4,5-benzenetetracarboxylic acid; it has a simple and easy preparation method, low cost, can be stable below 230 degrees, and has Strong third-order nonlinear optical saturated absorption performance, the third-order nonlinear saturated absorption coefficient β of the material is ‑4.55×10‑6m / W, and the third-order nonlinear polarizability χ(3) is 7.33×10‑8es , can be widely used in laser pulse compression (Q-switching and mode-locking) and other fields.