You Have a short to ground. Power takes the path of least resistance, and the fuse being your protection device blows when amperage is exceeded. trace the fuel pump wires looking for open copper wires. If you have a voltmeter near you (For S&G's) hook up the ammeter in series with the circuit, meaning pull the blown fuse out and put the ammeter wire pos and neg in two seperate holes. If the ammeter test wires and too thick to fit in the fuse holes use a paperclip and test off that. Doing this will give you the amperage which if exceeds the fuse rating (20A) mean short to ground. DO NOT PUT A HIGHER AMP FUSE IN! Like i said its a protection device which is protecting the fuel pump.
Trace all the wires making sure none are sliced or open. Use the volt meter again on OHMS, Disconnect battery (OHMMeter puts test with .5v so the battery will throw it off. Hook the Ohm meter up in series again and test for resistance. Remember when Resistance goes up amperage goes down and vise versa.
PM Me with any questions.
US Marine MOS: Diesel Mechanic
Fabricator/Mechanic Kool Kidz ATV