The petal-like CoO/CNFs composites was synthesized in situ by a simple hydrothermal modus. Electromagnetic parameters of the CoO/CNFs composites are further adjusted by adjusting the weight fractional number of the CoO. A rich interface of CoO is introduced into the carbon nanofibers to form a flower-like composite. This unique flower-like layered structure and appropriate electromagnetic parameters effectively enhance the impedance matching and attenuation capabilities. When the mass ratio of CoO to CNFs is 2:1, the obtained flower-like CoO/CNFs composites achieves a maximum reflection loss of − 34.62 dB with a thickness of 3 mm at 7.8 GHz. At the same time, the effective absorption area is 2.89 GHz (7.035–9.925 GHz). Therefore, taking into consideration its excellent microwave absorption properties, the petal-like CoO/CNFs composites may be deemed as a high-efficiency absorbing agent that satisfies the impedance matching and attenuation characteristics.