Carbon fibers, with different lengths, were used to modify the cement paste (CFRC). The effects of carbon fiber length on the microwave behavior of CFRC were studied by analyzing how the length affects the mechanical, dielectric, and microwave behavior. The results show that the compressive strength and flexural strength of CFRC increase first and then decrease with increasing carbon fiber length, such that the highest strengths are obtained at a length of 6 mm, which is attributed to the “bridge” effect of carbon fiber in the cement matrix. The electromagnetic interference shielding mechanism of CFRC is dependent on carbon fiber length: the reflection is dominant at lengths of 1 mm, 9mm, and 12 mm, while absorption is dominant at a length of 6 mm. The absorption loss is positively related to the real part of the complex permittivity of CFRC, while the relationship between the reflection loss and the imaginary part of the permittivity is unclear. The reflection of CFEC to electromagnetic waves is due to the mismatch of impedance, while the absorption of which to electromagnetic waves is due to the dipolar polarization, relaxation polarization, and space-charge polarization.