Fluorine-doped (F-doped) trench-assisted structure is proposed to improve the nonlinearity of photonic crystal fibers (PCFs). Through controlling only two parameters, three all-solid highly nonlinear PCFs with low dispersion slope and low confinement loss are designed. They exhibit all normal dispersion, two zero dispersion wavelengths (ZDWs) and one ZDW just at 1.55 μm, respectively. The dispersion slope of one ZDW just at 1.55 μm is 5.12×10-4 ps/(km·nm2), which is two order of magnitude lower than that of conventional highly nonlinear fibers. A nonlinear coefficient of 31.5 W-1·km-1 and loss low of 9.62×10-5 dB/km at 1.55 μm are achieved for this PCF.