Conventional spectrometers are bulky, and researchers have continuously made efforts in their miniaturization and integration in recent years. Among these studies, metalenses have attracted immense interest because of their merits of a flat shape and flexible regulation. Herein, we introduce a design of a polarization-insensitive metalens-based spectrometer that utilizes an off-axis high-efficiency broadband metalens in the wavelength range of 500-1000 nm. The demonstrated metalens consisting of nanopillars employs propagation phase and phase function optimization methods and can achieve spectral resolutions of 0.6 nm with efficiency as high as 77%. By stitching metalenses with different focal lengths, the functionality of the spectrometer can be expanded. Hence, a compact variable design with favorable focusing and dispersive properties can be achieved by one single component instead of traditional cascading optics, thus shrinking the volume to the millimeter scale and reducing cost. This research proves the potential for applications of metalenses in spectrometers as well as other consumer and industry products.