We present the deepest 850 $\mu$m map of the SSA22 field to date, utilizing a combination of new and archival observations taken with SCUBA-2, mounted at the James Clerk Maxwell Telescope (JCMT). The mapped area covers an effective region of approximately 0.34 deg2, achieving a boundary sensitivity of 2 mJy beam-1, with the deepest central coverage reaching a depth of σrms ~ 0.79 mJy beam-1, the confusion noise is estimated to be ~ 0.43 mJy beam-1. A catalogue of 850 $\mu$m sources in the SSA22 field is generated, identifying 390 sources with signal-to-noise ratios above 3.5, out of which 92 sources exceed 5σ. The derived intrinsic number counts at 850 $\mu$m are found to be in excellent agreement with published surveys. Interestingly, the SSA22 number counts also exhibit an upturn in the brighter flux region, likely attributed to local emitters or lensing objects within the field. On the scale of ~ 0.3 deg2, the 850 $\mu$m number counts are unaffected by cosmic variance and align with the blank field. In the deep region (σrms ≤ 1 mJy), the counts for fluxes below 8 mJy are consistent with the blank field, and the excess in the brighter regime is not significant. Due to the limited number of very bright sources and the insubstantial cosmic variance in our field, we attribute the fluctuations in the number counts primarily to Poisson noise. The SCUBA-2 850 $\mu$m detection in the SSA22 field does not exhibit indications of overdensity.