The linearity of microwave photonic link with direct modulation mode or external modulation mode is theoretically studied. The direct modulation short-range microwave photonic link, which is used direct modulation laser as the core device, has obvious advantages in linearity. Then the experiments are carried out to obtain key performance parameters through analysis of acquired signals under the two modulation modes, including the link loss, compression dynamic range (CDR) and spurious free dynamic range (SFDR). The dynamic ranges of fiber links with different lengths are compared, and the performance of the microwave photonic link with direct modulation drops significantly after the 10-km optical fiber transmission. In addition, considering the linewidth and chirp of directly modulated laser, dispersion effects for fiber links with different lengths are studied experimentally. The result shows that the microwave photonic link with direct modulation has high linearity for short-distance transmission.