The pine needle gall midge, Thecodiplosis japonensis Uchida et Inouye, is a newly invasive pest in China that mainly harms Pinus thunbergii and P. densiflora. The occurrence and damage caused by T. japonensis in pure stands of P. thunbergii were investigated, and the needle growth and needle compound content were measured. Based on the above steps, the growth dynamics of galls and chemical defense response of P. thunbergii to attack by the gall midge were revealed. The results showed that the adults of T. japonensis in Qingdao city, China, emerged from the end of May to late July, with a peak in mid-June. Needles of P. thunbergii began to differentiate in late June and stopped growing in mid-September. The length of infested needles was 60.17% less than that of healthy needles. On average, there were 9 ± 4 larvae in each gall, 22 at most and 1 at least. The number of larvae within a gall had no significant effect on the size of the gall or larvae. Compared with that in the ungalled tissues, the content of amino acids in the galled pine needle tissues increased by 40.83%, while the content of total polyphenols, tannins, carotenoids, total triterpenes, total alkaloids and other secondary substances decreased to varying degrees, which was favourable for the growth and development of the T. japonensis larvae.