3900 yuan. The calculation formula of interest is that interest is equal to the total loan multiplied by the annual interest rate. The total loan amount is 654.38 million, with an annual interest rate of 3.9%, which is 0.039 in decimal form. Substituting into the formula, it is calculated that 100000 times 0.039 equals 3900 yuan, so the annual interest of the 3.9-interest loan of100000 is 3900 yuan.