Two sets of multiple-color (B, V, R-C, I-C) light curves of PZ UMa were observed independently with the 2.4 m telescope at the Thai National Observatory and the 1 m telescope at Yunnan Observatories. The light curves were analyzed with the Wilson-Devinney program and the two sets of light curves produced consistent results, which shows that PZ UMa is a W-subtype contact binary with an extreme mass ratio (M-1/M-2 = 0.18). The basic physical parameters of PZ UMa were determined to be M-2 = 0.77(2) M-circle dot, M-1 = 0.14(1) M-circle dot, R-2 = 0.92(1) R-circle dot, R-1 = 0.43(1) R-circle dot, L-2 = 0.46(2) L-circle dot, and L-1 = 0.15(3) L-circle dot. The orbital period analysis of PZ UMa revealed a 13.22 yr periodicity, which implies that there may be a tertiary component orbiting around the binary system. The mass and orbital radius of the tertiary component were calculated to be M-3 = 0.88 M-circle dot and a(3) = 3.67 au, if the orbit was coplanar with the central binary system. It is interesting that the minimum mass of the tertiary was calculated to be M-3min = 0.84 M-circle dot, which means the tertiary component is even larger than the primary star and the secondary one of PZ UMa. PZ UMa is a late-type contact binary with stellar activity. The O'Connell effect appeared on its light curves when it was observed in 2016 April. However, the O'Connell effect reversed when the target was observed again in 2016 December. The changes of the O'Connell effect on such a short time-scale strongly support the occurrence of rapidly changing magnetic activity in this W UMa binary.