The work aims to establish interconnectedness between optical property and thermal property by studying the two properties of CVD diamond films of different weight prepared by DC arc plasma jet. Surface morphology, grain size, structure feature, infrared optical property and thermal property of CVD diamond films were examined with optical microscope, X-ray diffractometer, Raman spectrometer, Fourier-transform infrared spectroscope and NETZSCH LFA467. The optical property was closely related to thermal property of free-standing diamond films. The two properties mainly depended on content of N and non-diamond phase. As N content of the films exceeded 0.009%, N content was the key factor affecting both properties, and both exhibited linear attenuation trend. As N content was below 0.009%, N content had slight effects, grain size became the main factor affecting thermal conductivity of the diamond films, and grain size had slight effects on IR transmittance of the diamond films. In addition, C—H absorption was positively correlated with content of non-diamond phase, its law of influence on thermal conductivity and IR transmittance of the diamond films was basically consistent with that of N impurity. Thermal conductivity and IR transmittance of diamond films gradually increase with the decrease of N impurity content and C—H absorption coefficient, and the thermal conductivity is more sensitive than thermal transmittance when the N impurity content and C—H absorption coefficient decrease to certain level.