In this work, the adsorbent based on tannin (TN) and polyethylenimine (PEI) was prepared via a simple and green method (TNP). The characteristics of the TNP were determined using scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, and the results revealed that TNP was synthesized by TN and PEI, and Cu(II) was combined with the -NH or -OH groups of TNP. The effects of the n((TN)):n((PEI)), adsorbent dosage, pH, time, initial metal concentration, and temperature on the adsorption of Cu(II) were investigated in detail. The adsorption of Cu(II) by TNP was greatly affected by pH, and n((TN)):n((PEI)), = 1.5:3 was the optimum mole ratio of raw material to obtain TNP. TNP possessed excellent adsorption properties, and the maximum adsorption capacity of TNP for Cu(II) was 94.5 mg/g at 338.15 K. The adsorption process was in agreement with the Langmuir model and the pseudo-second-order model. Regeneration and reusability of TNP did not decrease significantly after four times of regeneration experiments. The order of competitive ability for the three metal ions was as follows: Pb(II) > Cu(II) > Zn(II). These experimental results suggested that TNP could effectively remove metal ions from aqueous solution. This study is of great significance for environmental protection.