An on-line detection method of chromium-free coating film thickness on surface of strip steel, characterized in that the method comprises the following steps: Step 1: selecting two water-soluble chemical substances which contain elements P, Ca, Ti, Ba or Sr and do not react with a chromium-free coating liquid, one of the two selected substances having an intensive characteristic spectrum and the other having a weak characteristic spectrum; Step 2: adding the two water-soluble chemical substances selected in step 1 into the chromium-free coating liquid and agitating them to be homogeneous, thereafter, fabricating a reference sample of the coating film; Step 3: using a ray emitted by an off-line film thickness detection instrument to excite the two water-soluble chemical substances so as to obtain the respective characteristic spectra of the two water-soluble chemical substances, thereby to have the coating film thickness of the reference sample; the coating film thickness being determined by means of the water-soluble chemical substance which has the intensive characteristic spectrum is taken as actual film thickness, whereas the coating film thickness being determined by means of the water-soluble chemical substance which has the weak characteristic spectrum is taken as measured film thickness, the difference between the actual film thickness and the measured film thickness is taken as a thickness correction value; by many times of such operation, obtaining many thickness correction values corresponding to the many measured film thickness, a correction function expression between the measured film thickness and the thickness correction value is obtained by fitting thickness correction values and measured film thickness; Step 4: adding the water-soluble chemical substance which has the weak characteristic spectrum into the chromium-free coating liquid, and using a ray emitted by an on-line coating film thickness instrument to excite the substance in chromium-free coating film of on surface of strip steel and to obtain the measured film thickness, and then using the correction function expression to obtain a thickness correction value, finally, actual coating film thickness in chromium-free coating film of on surface of strip steel is obtained from the measured film thickness and thickness correction value. Step 1: selecting two water-soluble chemical substances which contain elements P, Ca, Ti, Ba or Sr and do not react with a chromium-free coating liquid, one of the two selected substances having an intensive characteristic spectrum and the other having a weak characteristic spectrum; Step 2: adding the two water-soluble chemical substances selected in step 1 into the chromium-free coating liquid and agitating them to be homogeneous, thereafter, fabricating a reference sample of the coating film; Step 3: using a ray emitted by an off-line film thickness detection instrument to excite the two water-soluble chemical substances so as to obtain the respective characteristic spectra of the two water-soluble chemical substances, thereby to have the coating film thickness of the reference sample; the coating film thickness being determined by means of the water-soluble chemical substance which has the intensive characteristic spectrum is taken as actual film thickness, whereas the coating film thickness being determined by means of the water-soluble chemical substance which has the weak characteristic spectrum is taken as measured film thickness, the difference between the actual film thickness and the measured film thickness is taken as a thickness correction value; by many times of such operation, obtaining many thickness correction values corresponding to the many measured film thickness, a correction function expression between the measured film thickness and the thickness correction value is obtained by fitting thickness correction values and measured film thickness; Step 4: adding the water-soluble chemical substance which has the weak characteristic spectrum into the chromium-free coating liquid, and using a ray emitted by an on-line coating film thickness instrument to excite the substance in chromium-free coating film of on surface of strip steel and to obtain the measured film thickness, and then using the correction function expression to obtain a thickness correction value, finally, actual coating film thickness in chromium-free coating film of on surface of strip steel is obtained from the measured film thickness and thickness correction value.