The invention discloses a preparation method of a vanadium and nitrogen co-doped titanium dioxide dispersion liquid, and a rapid and continuous formaldehyde decomposition photocatalyst emulsion prepared by using the vanadium and nitrogen co-doped titanium dioxide dispersion liquid. The preparation method of the vanadium and nitrogen co-doped titanium dioxide dispersion liquid has the advantages of low cost, simple process, less heating process and less energy consumption. The vanadium and nitrogen co-doped titanium dioxide dispersion liquid adopted by the photocatalyst emulsion plays a role in catalysis in visible lights, so the photocatalysis efficiency is greatly improved. The periphery of an amino-terminated hyperbranched polymer in the photocatalyst emulsion contains a large amount of amino groups, so the dose of a reaction of the emulsion and formaldehyde is large, generated polymer is stable, and reverse release of formaldehyde is avoided. The photocatalyst emulsion can directly react with formaldehyde, can decompose formaldehyde and other harmful gases in air in a dark area in a dark period, can greatly reduce the formaldehyde content of plates from the headstream, and can continuously improve the quality of air.