Sensitivity of magnetometers that use color centers is limited by poor photon-collection and detection efficiency. In this paper, we present the details of a newly developed all-optical collection combined frequency-modulated microwave method. The proposed method achieves a high sensitivity in static magnetic-field detection both theoretically and experimentally. First, we demonstrate that this collection technique enables both a fluorescence collection as high as 40% and an efficient pump absorption. Subsequently, we exploit the optically detected magnetic resonance (ODMR) signal and quantitative magnetic detection of an ensemble of nitrogen vacancy (NV) centers, by applying a frequency-modulated (FM) microwave method followed by a lock-in technique on the resonance frequency point. Based on the results obtained using all-optical collection combined FM microwaves, we verified that the sensitivity of the magnetometer can achieve approximately 14 nT/√Hz at 1 Hz, using a discrete Fourier transform detection method experimentally. This method provides a compact and portable precision-sensor platform for measuring magnetic fields, and is of interest for fundamental studies in spintronics.