In May 1941 Sun Bo-Xian probably served in the 21st PS of the 4th PG. This unit was at the time equipped with Polikarpov I-153s.

On 21 May there was a duty flight of eight I-153s from the 21st PS and one from the 29th PS at Xigucheng aerodrome commanded by Captain Chen Sheng-Hsing. During the day they scrambled to intercept 27 Japanese bombers over Lanzhou. The Chinese fighters flew in two formations. The first of five fighters was led by Chen Sheng-Hsing while the second of four fighters was led by Chang Kuang-Yun, vice-commander of the 21st PS. The first formation claimed a bomber shot down and it was shared between Wu Guo-Duan, Sun Bo-Xian, Hu Nai-Wu, Zhu Huan and Chen Sheng-Hsing. This was the first Chinese claim with the Polikarpov I-153.
The Japanese bombers were G3Ms from the Mihoro Kokutai. One G3M was lost and another returned after suffering 70 hits.

Sun Bo-Xian ended the war with 1 shared biplane victory.

Claims:

Kill no.	Date	Number	Type	Result	Plane type	Serial no.	Locality	Unit
	1941
	21/05/41	1/5	G3M (a)	Shared destroyed	Polikarpov I-153		Lanzhou area	21st PS

Biplane victories: 1 shared destroyed.
TOTAL: 1 shared destroyed.
(a) Claimed in combat with G3Ms from the Mihoro Kokutai. One G3M was lost and another returned after suffering 70 hits.

Sources:
Soviet Fighters in the sky of China, Part V - Anatolii Demin, 2001 Aviatsiia Kosmonavtika 1 (translated by George M. Mellinger)
Additional information kindly provided by Raymond Cheung and Erich Wang.