In light of the two types of relationships (complementary or substitutive) between first-party and third-party contents, this paper constructs a theoretical model to delve into the impact mechanism of different investment types of first-party content on the price and quality of first-party content and third-party content, as well as the mechanism influencing the revenue of digital content platform and third-party content provider. The main research conclusions are as follows: (1) Compared to platform not investing in first-party content, when investment efficiency is relatively high, platform investments in complementary first-party content can lead to an increase in the quality, price, and demand of third-party content; on the other hand, when investment efficiency is relatively low, it can lead to a decrease in the quality, price, and demand of third-party content. Meanwhile, investing in substitutive first-party content always leads to an increase in the quality, price, and demand of third-party content. (2) At appropriate times, platform adopting a strategy of high-quality, low-price complementary first-party content or low-quality, high-price substitute first-party content is more conducive to the sustainable development of the platform ecology. (3) Investing in complementary first-party content is always advantageous for the platform, but not necessarily beneficial for the profit growth of third-party content providers; on the other hand, investing in substitutive first-party content always leads to the growth of profits for both parties. Moreover, greater competition intensity favors the growth of profits for both parties.