| Document and Entity Information | Oct. 04, 2021 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 4, 2021 |
| Entity Registrant Name | ALX ONCOLOGY HOLDINGS INC |
| Entity Central Index Key | 0001810182 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity File Number | 001-39386 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 85-0642577 |
| Entity Address, Address Line One | 323 Allerton Avenue |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 650 |
| Local Phone Number | 466-7125 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | ALXO |
| Security Exchange Name | NASDAQ |
| Former Address | |
| Document Information [Line Items] | |
| Entity Address, Address Line One | 866 Malcolm Road |
| Entity Address, Address Line Two | Suite 100 |
| Entity Address, City or Town | Burlingame |
| Entity Address, State or Province | CA |
