Wide-angle, high-efficiency, wide-band, and ultra-compact focusing blocks are crucial for implementation and future evolution of passive millimeter-wave focal plane array imaging systems. The spherical or doublet metalens can attain high-efficiency, wide-angle field-of-view (FOV) but suffer from fabrication difficulties, complex assembly, and low compactness. Here we present an efficient single-metalens design capable of performing high-efficiency diffraction-limited wideband focusing over a wide-angle FOV. This single-metalens design can greatly mitigate the Seidel aberrations by a rational allocation of amplitude-phase of the electromagnetic waves. A proof-of-concept metalens at millimeter-wave band (33 GHz-37 GHz) confirms the validity of our design.