The reproductive investment strategy is important for the survival and reproduction of parents. Yet, due to extra-pair copulation behavior of female, socially monogamous males may end up making reproductive investment in some offspring that, in terms of genetics, are not theirs. Our study centered on varied tits (Sittiparus varius), a socially monogamous species with high proportion of extra-pair paternity. We compared male and female reproductive investment (nest defense and brood investment) and tested if extra-pair paternity, mate’s body condition, and mate’s investment affect their reproductive investment. Results showed that, the nest defense strength of males and females the difference was non-significant; and on feeding frequency, the difference was clear: females were significantly higher than that of males. With or without of extra-pair offspring, mate’s body condition and mate’s feeding frequency were not significantly related to parental reproductive investment. On one kind of reproductive investment, the strength of nest defense, correlation was detected between social mates. The relationship between reproduction investment of the bird’s parents and extra-pair paternity was non-significant, possibly as a result of males’ current reproductive tradeoffs. In summary, the existence of extra-pair paternity does not reduce the reproductive investment of males, the behaviour of mates affects the nest defense strength of individuals, and it is possible that this is a function of the shared environment, the exact reasons for which still need to be further explored.