We experimentally demonstrate a high-average-power polarization-maintaining (PM) fiber-integrated chirped pulse amplification (CPA) system operating at 1.06 μm with almost fully fiberized design by using chirped fiber Bragg grating as stretcher. Before compression, average power of 425 W is achieved in the fully fiberized amplifier chain with corresponding optical to optical efficiency of 81%, polarization extinction ratio (PER) of ∼13 dB, and M2 factors of ∼1.2 both in the x and y directions. The mismatched dispersion between pulse stretcher and compressor is compensated by optimizing the value of nonlinear phase shift at an output average power of 425 W. After compression, femtosecond pulses with average power of 300 W, repetition rate of 80 MHz, and pulse duration of 344 fs are achieved with a compression efficiency of 70.6%. The corresponding pulse energy and peak power are calculated to be 3.8 μJ and 11 MW, respectively. To our knowledge, this is the highest average power ever reported in the PM fiber CPA system. Further power scaling is limited by mode deterioration, which causes sharply deteriorations of the beam quality and PER. The mode deterioration has threshold power but without temporal intensity fluctuation, which may be attributed to the quasi-static mode degradation induced by photodarkening.